can pokemon black and white 2 be play in the original nintendo ds? if yes.... then yes.... some functionality will not be present though like 3d off battle or something

I could definitely see a third version (or a second pair, akin to B2/W2) having new3DS (I think I might write it like this, to make it distinct from just "new 3DS") functionality available, but I don't think it'll be exclusive to it.

Kind of like how the original Pokemon games (up to, but not including, Crystal) had colour coding for GBC, but still worked on GB.

No I hope not, they'd loose millions and millions of game sales from the old 3DS user base

I expect this to play out like the fifth-generation Pokémon situation.

Exclusive features (and a smoother game overall) for New 3DS, whilst maintaining compatibility with the 3DS.

Just look at Pokemon Gold and Silver. They were designed to take full advantage of the GBC, but also played perfectly fine on the GB in black and white. Same will go for any future Pokemon game releasing on the 3DS consoles. Nintendo is a little goofy sometime, but they aren't stupid.