You know, the most logical step for the next game would be an Akaviri invasion. The Akaviri HATE dragons (even if technically their dragons are different from Skyrim dragons), at least as stated by The Mysterious Akavir, and it is also possible the Tsaesci and the Ka Po'Tun tried to become dragons themselves with varying success. Now that the dragons have returned in Skyrim, it would be only logical that they would try to get to them, and an outside invasion would be a great way to shake up the war between the Empire and the Altmeri Dominion and give way for a chance to re-forge the empire.
Not to mention, it could actually work well with the Black Marsh location, as it is indeed on the eastern end of the continent (where the Akaviri invasion would come from) and the reportedly reptilian-like Tsaesci might want to start their incursion on a land with familiar inhabitants and possibly suitable environment. It would also explain why other races would be there in number, as part of the defense effort, and it would justify all races for character selection since, well, there would be enough members of there race there to justify their presence.
For me it sounds absolutely logical for the next game to go this way.