The trees as pets thing is a damn dirty hack so that we can have fruit-bearing trees. Actual, proper trees in DF can't be made to grow fruit, so we end up with wooden sheep and blackbox reactions. I suppose making a special material for them that could be converted into a log and some fruit would work, but it's still a hack, and you have to cut down the tree to get the fruit.
Making rabbits and parasprites adopt is my kind of evil. Based on some of the toys, it might be an idea to size up/include owls, chipmunks, raccoons, parrots and hedgehogs for use as small pets. I'd get to meet my weird quota with a reaction to convert sheared hedgehog quills into blowdarts. There's no way to make specific animals have blood feuds with other species. The problem, for me, is making these animals useful, like, is there a way to make them do simple hauling tasks, or will letting them speak(hopefully "utterances") allow them to make friends and give positive thoughts? I suppose having preference-free nobility would be pretty sweet. If only I could assign tanks to the fortress guard...
On vanilla plants, my thinking on this was just replacing "dwarven" with "cave" or the base plant name, not actually replacing the vanilla content. Really, unless we see dwarf ponies and their farms in the show, we've got no basis for making replacements, but I suppose it could be a fun thing for flavour. Goatish is a good idea, maybe I'll add upper cavern biomes to mountain goats to justify that, with cave goats lower down.
Poison joke is actually already in, it's a dye plant(grey, inspired by vulpinus, a blue flower that makes grey dye in real life). Syndromes will have to wait until the base game is updated, and it's difficult to include syndromes that are funny, rather than harmful, though hopefully the were-system will allow for funny reactions like changing size, gender, or into a bunny temporarily. Hopefully the were names won't be hard-coded, because were- means man. Equo- or something would be a fair substitute.
Adding multiple creatures to an entity makes the entity pick from them randomly, hence the combined civs. Sorry, this mod really is held together by chewing gum and the power of friendship, Dorf Fort and MLP are two very distinct flavours of batshit crazy and getting them to coexist takes some compromises.