Well, I've been taking a break from the game as well, for about three months now. But that's mostly just boredom with MoP .. I know I'll come back when WoD comes around.
But while I know what you mean especially after playing ESO for a bit (which is a great game for solo play, except for when you want to do dungeons), there
are of course ways to play solo.
You could work on getting exalted with old factions for titles and more, or complete some old quests for achievements.
There's also always the battle pet system... While there's pvp battling, you can always just battle wild pets too; collecting all the wild pets and pets from old raids/factions/quests can give you something to do too.
You can solo farm old raids and dungeons for transmog and mounts (and achievements while you're at it)
Maybe make a new alt if it doesn't bore you too much; maybe a new class, or if you've only played one faction to max level, try out the other. Questing can be done entirely solo of course, though it can be slower than dungeons IMO.
Maybe collect all the rare hunter pets, or unique looks? That often involves a lot of waiting around though, and I don't know how many of them you have.
Level professions, even though I find it boring.. It can be done alone.
Can't really think of anything else, though
Also most people use wowhead nowadays instead of thottbot
