I loved the AoW3 Terrain preferences, having to use other races than your own simply because of Goblins hating arctic areas or Frostlings hating vulcanic areas.
If you wanted to conquer/defeat other fractions on terrain your race doesn't like you had to either get yourself one of these "hearts", be necromancer or use another race.
Ofcourse you can't bind forms to the different terrains as it was made in AoW3, but you could make a 3d race creation field for it.
That would also enable players who dislike having to give a body trait for things like underground adaption to give their race a terrain adaption without having to give up the body trait.
(I hate not being able to create sneaky cave dwelling rats and goblins)
Only downside to this is you'd have to make several more adaptions, once for each terrain, so everyone can choose one fitting to his playstyle, plus maybe one allrounder adaption for those who don't want to specialise themselves.