I like the ideas going here.
I suggest using Sandbox with the Guardians blocked off. It will give you the room you need.
I am against Scorpion v Scorpion because those battles are usually determined by first shot.
Wraith v. Wraith is good, though it doesn't happen very often in BTB, but it would give people practice aiming with the Wraith, which is useful. You could make the wraiths fight out in the sand, so they could maneuver on uneven terrain and such. (However, Wraiths could cheat by interfering with other battles)
Chain hog v. Chain hog would be sick. Move stuff around on the base floor of Sandbox so they could use cover and the like. It would allow drivers to get creative and see how well gunners can keep up with maneuvers. I also suggest there be bubble shields placed on the map to allow the clutch bubble shield saves. Perhaps even placed near the spawn.
Banshee v. Banshee is definitely something I'd like to see. The Sky Bubble would be cool, but it might be too open. Most Banshee fights involve both teams trying to help their banshee survive, and the Banshees are often forced to break off.
Hornet v. Hornet is good, but I think Twisted has the better plan with incorporating hogs into the mix as well, as a Hornet will often have to deal with hogs as well.
I like the 2 v 2 BR battles. Perhaps you could even consider moving up to 3 v 3. The team that team shoots best is most likely to win, unless it devolves into multiple 1 v 1's and the best BR's win out.
Sniper v Sniper may be important, as it happens quite a bit at upper keyhole, but that's just one map.
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