Of all the new biomes, I think roofed forest would be the best one/most unique for battles.
If people want a jungle & taiga hybrid, I'd be willing to give up some UYP land for it, though there'd have to be a no nether portal rule since I'm very obsessive about portals near me.
Personally I'd prefer a roofed forest and extreme hills hybrid, something like this:
As for how the new bg works, remove flags, remove most zones: either make it anarchy, make zones really big or make some zones where you can build and conquer things but most of the space is unclaimed. I'd like it if battles weren't as planned, allowing for sorts of sneak attacks/guerrilla warfare. Just ideas I have