Why did Bethesda choose to make two different story lines for this quest? I suppose we don't know the full story yet, but in the lore it is presumably going to have to go one way or the other. So in light of that, why couldnt the imperial quest line been based around base defense against the rebelling stormcloaks. And if you choose stormcloaks it is more based around offensively assaulting the towns. Or visa versa, it doesn't really matter in my opinion.

I guess whatever side the Dovah sides with should win the war although. But maybe if only one side was chosen by the lore to win and the Dovah chose the alternative side, the winning side could be using some power,weapon,tactic, to make the fights unfair. Maybe the thalmor could be the cause for one side to win.

I don't know maybe my idea is too black and white. I just don't see the point of my characters fighting and winning a war that may have a different victor one day.

The world was the main focus, everything else took a backseat.

If they don't add an import feature into the next TES, if there is one, it'll probably take place so far into the future it won't even matter who won

Something's going to happen to make the choices nonimportant. One of them won but history will never remember which it was.

I'm thinking that regardless of which side wins, the Thalmor will attack pretty soon afterward.
The next game will probably have something to do with Aldomeri Dominion vs. The Empire. Be it as the main quest or the results of it would be the foundation for the story (years after the ending of the war)

I wonder how they'll handle the two fates of the Dark Brotherhood though, one seems like

Well isn't that basically the problem with all choice based RPGs? Like in Fallout 1, we could've chosen to destroy Shady Sands but in Fallout 2, that choice is null and void. It's just a byproduct that can't be fixed unless you want to wait an extra 4 years for Bethesda to make a Mass Effect style save import system.

It will most likely play out as either Ulfric won but the imperials just launched another attack and completely took over Skyrim after he started to oppress the people and forced the jarls to do whatever he says, even if it meant to kill innocents.

All records will probably be destroyed in something catastrophic or something. When the smoke clears you will be left with conflicting accounts of what occurred in the chaos.

As others have said, it will probably work out that either the empire falls anyway or there's an imperial resurgence and some new guy re-conquers everything, or everything falls to the Dominion for a time. No one will remember this period except for there being dragons in Skyrim for a while.

I don't think the one that you kill the brotherhood will be the lore friendly one I doubt Bethesda would just kill off the brotherhood.

OR... the dead body confirmed his "bury my mother" story and he was released, probably only to kill everyone afterwards.

It will probably be written that somebody killed off the DB but a few escaped (Babette and the Redguard) and rebuilt the DB.