What about taking ideas from the ashran mini games and making them into BGs? Or Hutball from the old republic?
Something like this might be tough to pull off but imagine a flag map that required you to throw the flag to other players and then a skill shot through a goal?
Been thinking on this for quite a while.
I would love to see a 12-15 man BG with 3 person teams that is similar to a bounty extraction game. I think this would be a relatively simple add overall. You pick up an item like the ball in Temple of Kotmogu but when you die it drops it. It could be similar to arena as well in the idea that once you die you cannot respawn unless you are resurrected by a teammate.
Could be a lot of fun imo but would need a medium/large sized map for search and clues with different spawn locations. Add in PvE element and it could be a good 20-30 minute game.