Although lore-wise this would be acceptable, as someone mentioned it seems as if they (the devs) are trying to move away from the septim line, forget the oblivion loopholes for a second and all the possible lore answers and lets look at oblivion the game for a second, more specifically lets look at the main ideas expressed in the game, the game keeps trying to stress that Martin was the only heir possible (we all know that this is not necessarily true, but lets keep following the main ideas) Ocato did not know of any other heirs nor did anyone else in-game this also stresses Martin's cruciality, if we accept that the devs were trying to close the septim lineage then we can accept the idea of closure which the avatar of akatosh represented in the end, as well as the reiterated phrases of 'who will rule over the empire with no heir'.
Suffice it to say that all this points to one main idea, the devs are trying to get rid of the Septim lineage, IMO the devs are planning something, they were planning something for TES V since oblivion just because of these small hints, the only answers anybody will get is in TES V, in other the next game will answer alot of mysteries left in oblivion.
I wonder also if Keyes' TES novel(s) are a clue to TES V. The first novel is said to be set 40 years after the Oblivion crisis, so maybe the next emperor hasn't been born yet at the close of the Third Era. He might come from any province, though I think he's more likely to be Nord, Redguard, or Breton. (You could even have the next ruler be an empress, but I'm not sure that the devs will choose a woman as the ruler in TES V.) Come to think of it, Keyes has a tradition of writing about strong women in his series "The Kingdom of Thorn and Bone", so I guess even that isn't impossible.