Basically he wants Digimon you select as the leader to have an effect on the battle background. Each mon would have an icon on its profile that shows what kind of background it would be
For once I support a suggestion from you Backgrounds are kinda bland right now to be honest.
Maybe make it so that only SP or above mons have this effect, or BLB mons so its more "special". That way devs could probably put in more unique backgrounds rather than just a "leaf" background for nature mons or "lava" background for dragon ones. LRs having unique backgrounds is probably more likely though.
Perhaps for the VS digimon, it could be like a scene of them fighting? Probably not enough space for that though