Hm, some good points. I'll break the ones I like best down:
1. Non-dwarves needs:
Dwarves are usually easy to please; give 'em a bed, food, booze, and an awesome dining room and they're set. Other races might have more, fewer, and/or different needs. Many aboveground animalmen, as well as elves and maybe humans, would need some sunshine to be happy. Amphibianmen would need abundant fresh water to not be dehydrated. Some taller races, like some animalfolk and maybe humans, might need more headroom. Some other animalpeople and kobolds would need meat to eat; goblins, bearpeople, and so forth would prefer meat; and some other animalpeople would prefer fish. Without these kinds of needs being met, these races would lose happiness, or maybe their lives. On the other hand, some races might have an easier time with upkeep. For most animalpeople, having a specific place to sleep of a couple tiles per tribesmember would be a great luxury that they're not familiar with, beds optional. Kobolds and smaller animalfolk would be able to subsist happily on rodents or even insects (for frogmen, lizardmen, etc). Some animalmen could/would graze rather than eat. Elves might be perfectly fine with no furniture to call their own as long as they have access to the trees they love. And so forth. Basically, someone wanting to accomadate additional races would be exchanging certain needs that the new race lacks for certain others that they add, in addition to exchanging new abilities for social intrigue.
2. Semi/Megabeast Allies
Basically, the idea of having a semi- or megabeast ally itself (or BE allied) to your fortress would be an awesome development. I'd like to see it. However, for most, such as titans, it'd make sense for this alliance to follow the taming-and-training used on dragons and the like for now, until such time as the way you tame dragons changes. Semimegabeasts, on the other hand...They would be as interesting as any other race to add to the game.