I would definitely recommend getting out of the garrison if at all possible; not only are they subject to the same lag that Stormshield and Stormwind/Orgrimmar are going to experience, the phasing of them makes them an instance. Enough people logging on combined with potentially unstable post-upgrade server hardware means that "Instance Not Found" is very likely to be the refrain of the day, which means that you're going to go through two loading screens plus a pause (the garrison loading screen, a pause while the game realizes it can't put you there, and another loading screen while it ports you). I know the tech and the behind-the-scenes of the garrisons have improved since the start of WoD but it's still super obnoxious. Me? I'm gonna put all my alts in Exodar/Silvermoon, Shattrath, Dalaran, or Shrine if they aren't there already. It's too bad all of those areas are CRZ-town nowadays, but they should still be calmer than Eastern Kingdoms, Kalimdor, and Draenor. Darnassus might also be an okay city for my primary server, since we're heavily Alliance-dominated (but again, CRZ so that's harder to predict).
The day/night before the 7.0 patch itself drops I'm going to go through all 50 characters (god, why am I character capped) with an emphasis on the... 30? level 100s (GOD WHY DO I HAVE SO MANY LEVEL CAPPED CHARACTERS oh right for the stupid love rocket s i g h). The vast majority of them have a Dwarven Bunker/Warmill and a Salvage Yard at least at level 2 (most have level 3 but the eight alts that hit 100 after the end of April didn't get their garrisons upgraded to level 3 because I wasn't certain I could make the 5k+ costs of level 3 garrison+buildings back before the prepatch hit and all the gold from garrisons gets so severely hit). Any Salvage Yard boxes and bags that haven't been opened GET opened.
Any transmog from the boxes that I don't have that can be used on characters on that server gets soulbound (I've been using
Legion Wardrobe and
MogIt for this), any transmog that's potentially "worth something" (anything especially pretty or unique looks that can't be obtained from quests or more than three other sources) gets held onto and/or posted directly to the AH (depending on time constraints), everything else gets vendored or disenchanted.
All Iron Horde Scraps get turned into the daily Scrap->Weapon token, Orgrimmar Axes, Stormwind Shields (and then mailed to a Horde character to turn into Axes), and anything that can be afforded (Thunderlord Sabatons on Alliance, mostly doesn't matter Hordeside), in that order. All those pieces and all remaining follower ilvl upgrades get vendored, as ALL those things are losing the vast majority (or all) of their vendor value in the patch. Any remaining scraps get used on subsequent dailies if the character's bunker/mill is level 3 (you still get like 15.4g from turning in the quest on PTR and beta, and like 3% of level 100 on beta lol), and discarded otherwise.
All missions (naval and follower) get collected, but none get sent back out as even 100% success chance missions that are in-progress when a character is copied over are failing on PTR/beta right now so it's just a waste of resources/oil. That may be fixed (though I suspect it's low priority) or it may be a pure character copy issue, I dunno. The exception will be if someone procs a Blingtron's Vault just to spite me in case the issue gets fixed AND in-progress missions don't get their resources changed. I'm pretty sure that's gonna happen cos I haven't seen one across 35ish characters with garrisons in months.
All Garrison Resources get spent unless I can think of a specific need for them. If I've got professions still in progress, resources will go to mats for that, but otherwise they'll be spent on the sacks of gold at the trading post. It's probably not the most potentially lucrative course but it's more consistent than trying to sell Draenor profession mats in an environment where they will probably be in glut (much as Pandaria ones were when 6.0 dropped), and the gold sacks will be removed in 7.0.
On servers where characters are getting deleted once all their potential transmog gets put into my wardrobe and there aren't any additional characters on that server or their faction, any remaining gold is spent on the AH or on vendors for looks I still don't have. If time permits (and I may end up doing this a few days in advance instead of the day before to make sure time permits), I'll sit around on them for two hours to run out the buyback timer, as there has been talk of stuff not getting added to the appearance collection 'til it's well and truly soulbound to you. That's not how it's working on beta right now (at least according to the chatlog), but better safe than sorry. Likewise, any characters with enough honor or conquest to buy ANYTHING moggable will, because that's less pvp tokens or whatever I have to grudgingly collect later on and I'd rather have a piece of transmog than the 35s/point that the honor conversion grants.
All my shirts find their final homes. Whether that'll be my rogue (where they're being stored now) or one of my old mains or just scattered around, I'm not sure yet, but the shirts won't ever be deleted even if they are account-wide. They're too important. So I gotta figure out where I want them once they end up soulbound, lol.
Then all characters are taken to a "safe" location. It's a bit of a shame that all the places with void storage access are likely to be super crowded (Shrine will probably be the most stable but even that's gonna have a lot of folks I think. Stormwind/Orgrimmar may be okay depending on server population since those aren't CRZ'd), but a lot of my characters don't have void storage and I'm reasonably sure that items in void storage are automatically added to the appearance collection (though that's one of the things I didn't actually specifically test and now I'm not sure what I could test it WITH). Actual location per alt doesn't super matter as I have the Grand Expedition Yak so proximity to a transmogrifier (to turn off helms and cloaks since the hiding of those is temporarily stripped and also some alts are using transmog that's gonna get turned into mail or plate and will need to be re-mogged with the leather/mail versions of those looks) is not super important. The exceptions are going to be my main, who will be in Stormshield to purchase the CM weapon account-wide token (though I may play it safe and put her somewhere safe and just wait on that, we'll see), and my paladin, who will fly out to the Veiled Stair to prepare to pick up the two legendary cloak looks I no longer have in any of my bags (tank and healing). She could technically pick them up prior to 7.0 since the epic cloak does appear to be rewarded retroactively since they were quest rewards (while the legendary is crafted), but... I dunno. That's another one that will depend on how I feel at the time, haha.
After that, make a backup of my WTF and Interface folders and download any necessary addons that have a Legion update available already (some already do as they're both beta/ptr and live-compatible and are in double development). Remove any addons that don't have an update available AND are non-vital, make note of both those and the ones without an update that ARE vital, and do any necessary set-up on new or existing addons (something to shrink the Order Hall "TitanBar" is looking like the most vital but that doesn't have to happen for 7.0 proper). Once we get a drop date for the prepatch I'll start seriously looking into 7.0 addons and preparing for the drop, including figuring out what sort of weakauras I need to set up for 7.0 MM and either BM or whatever I've decided on for my proper Legion main (raid composition permitting, since I still don't know and thus haven't sent Naha a PM about it and am probably pretty far back in the queue).
Then head to the guild website and update our recruitment widget to include Demon Hunters and Survival under "melee dps," activate the "Demon Hunter" class group for our forum class colouring, edit the Tank Discussion forum's text to indicate there are now five tanking specs, and then wonder why I didn't do the non-widget stuff sooner. Maybe I should do that now, actually lol
On the day of the patch itself, log onto each alt in turn, backwards from my main (as my main is using transmog that's getting turned into plate in five of her nine transmoggable slots and will need my others alts to provide the BoE versions so I can re-transmog it), turning off or re-updating addons as indicated by any LUA errors that might be spit in my face. At least two alts who are slated for deletion will be deleted to make room for two Demon Hunters (Alliance and Horde), though the others can wait a bit to be sure the wardrobe doesn't get reset the way the pet collection did (I still don't have a Spirit of Competition because it got eaten in that fiasco and I couldn't get it restored ;_;). Characters that desperately need bag space can delete some of their (non-essential) transmog but a lot of that can wait for later, less hectic days. Use any upgrade items for Draenor profession items on them to gain the additional skins assuming that's been fixed by the time 7.0 drops.
As I hit Scarlet Crusade and Korgath, open a second copy of WoW to delete the placeholder characters on my RAF account that have been holding my preferred names and create the characters on those servers—Demon Hunters if the pre-order DH bonus happens at patch drop, new placeholder level 1s if not (so I don't have to worry about the RAF account anymore). If for some reason the two names don't work out (get sniped or are still locked to the RAF account or whatever), this step can be skipped 'til I figure out different names.
Pick up the Legendary Cloaks on my paladin and the CM gold weapon token on my hunter. Then, get my hunter somewhere safer than Stormshield to set up any additional addon shenanigans she needs to deal with and re-practice the 7.0 MM garbage until raid time.
Speaking of raid time, all this patch day planning goes horribly awry if servers don't come up til like, 4:30 PDT (or later) because raid starts at 5. If time is running short, all the rest of the horsing around has to be put on hold in favour of prepping my main for raid. If time is REALLY short, I may even have to use a different transmog 'til I can get my Soldier's set back D:
If raid goes well(-ish, and a lot of that is dependent on how much prep the rest of the raid has done as well as whether our raid comp and strat can even work on mythic Archimonde anymore) and I haven't been up all night/day I can finish anything I ran out of time for pre-raid and possibly look into invasions, if there's something there interesting. To be honest, I haven't looked into invasions at all so I'm not sure how that's gonna work out or if I have to prioritize it (time-sensitive transmog a la the teal Judgement) vs finishing quest transmog or running old raid instances or actually doing non-WoW things or whatever.