I think vanity pets could be a neat idea, if they implemented them carefully. IMHO they would have to be on the small side, no big obtrusive critters with flashy animations. And I'd like them to be fairly ordinary and, dare I say, pet like. No mini floating eyes or tiny beastmen! I think ones like these would be fun:

Dodo Chick
Young Dormouse
Opo-Opo Follower
Dwarf Coblyn
Befriended Spriggan

As for framerate and lag and such in cities, well, I dunno. But other games manage it just fine, and XIV is getting a snazzy upgrade, so I think they should be able to handle it too. Just my 2 gil, though.