Petopedia would profit really much from this. In my opinion this guides would be really useful:
-HowTo: Taming properly (how to tame a elite mob with max level without dying)
-HowTo: Getting rare-spawnes (how to set up addons and makros for this purpose and hints)
-Best pet families for dps (a guide that does the math and shows some neat graphs and formulas)
-Best pet families for soloing (same here)
-Best pet families for PvE (same here)
I would even write some of them myself. Since I'm a mathematics student here at germany I think I'm quite good for this

However, I'd like somebody to check them for grammatical errors since my english is just a foreign language to me.
2. Pics of Spawn locations
Yes, I know WoWhead offers this and that there's a direct link for every pet. However. Named pet's (for example the spirit beasts) should get a small graphic that shows the spawn locations for them.
3. gif Animations
WoW Modelviewer offers a nice option for this. gif Animations of the different stand and attack animations of every pet model would be really really nice. Yeah, it's quite much work but it would help deciding which pet to take (for many people) since at least for the user it's better to have a quick look than have to test-tame one ingame or open up modelviewer by himself