Actually, in terms of ES 6...I think Stormcloaks winning is actually what will happen.
Considering Imperials are based on the Romans...
And bethesda pretty much following roman history...and during the time of skyrim, that is pretty much 100% accurate.
The empire will most likely be gone or shattered in ES 6 (think eastern roman empire/byzantine empire...so a remnant of it will likely still remain)
AND the big clue. You assassinate the emperor for the empire...and there is a near 100% it is lore accurate (otherwise they wouldn't put that HUGE plot in)...which further goes in line with Stormcloaks winning...
So in truth, choosing empire is NOT lore friendly. I don't agree with the stormcloaks, HATE Ulfric (he looks, sounds and acts like a complete douche)...but it is a near 100% likely hood that is what will be shown in ES 6 is stormcloaks winning or half/half at the very least.
(edit: As for barbarians...I imagine stormcloaks can be considered that (nords)...considering barbarians (nords) were a HUGE part of the roman (empire) army...even the environment is sort of similar...bethesda is just lazily copying roman history into ES...but hey, I love roman history...so not a bad thing to me. But makes it VERY obvious what is going to happen. So no point in choosing anyone but stormcloaks if you care about lore...cause empire will either 50/50 Skyrim or lose. Depending how much bethesda uses real history.
Except the thalmor...not sure who that can be considered as. A wildcard? Any ideas I have, seems maybe pushing it. If considering they have asian influence (ESO)...and bethesda takes that...probably mongols or something...but to me, that seems pushing it too far right now with what I know of thalmor...I'm sure bethesda based them on someone in history. They did with the Imperials (roman obviously) and nords)