They could maybe make one or more campaign-only DLCs. As in what we thought V&V was gonna be. There could be one for Euro civs and one for Asian ones, and then a NA DLC which finally adds a Mayan campaign + at least 3 new civs. Or just a single DLC for with campaigns for all civs without one and after that we could get normal DLCs again.
Yes, anything is possible…we have Mayans, Romans, Magyars, Jurchens, Khitans, Chinese, Koreans, Japanese without campaigns…they have enough to make a The Conquerors/Forgotten 2 if they wanted… 
All umbrella Civs should be reworked to something specific, the Slavs make no exception.
They should either be renamed/readapted to (Kyivan) Rus or split/expanded into several relevant Slavic kingdoms. An ethnicity can’t be a Civ anymore.
I would argue that the current Slavs mainly represent the Rusian kingdoms rather than the Balkan ones so for that I’d stick to the former and leave the latter to a dedicated DLC. So if they had to be split I’d imagine something like Kyiv/Moscow/Novgorod.