1. Thaumcraft
2. Dynamic Trees
3. Traverse
4. Novam Terram
5. Terra
6. Vampirism

There are probably more biome-adding mods I can think of, and I'll add them here as I find them.

Thaumcraft support is already added in dev
Traverse support is planned and will be added eventually.
Any other mod support that was in 1.10 will be readded if there are 1.12 versions of the mods.
Dynamic Trees - This can't happen in RTG, because it requires a client-side component.
Terra is discontinued and was replaced by Novam Terram, so Terra support isn't going to happen.

Hm, what about Ice and Fire? It has one biome, a glacier biome.

Yep, we had support for that in 1.10.2, so that will be migrated across as well.