Competitive mode that only allows for main-base spawn. When all flags is captured, every flag goes neutral, everyone spawns back at main-base and the team that captured all flags get one win. One round lasts 30 min. Fits well on metro conquest. Would like that tested!
Judging from what's been made with BC2 and the possibilities of a fully fledged out Rime, I'd say new gamemodes/importing campaign maps should be possible. Regarding BF4 maps, someone else'll have to answer.