Do we know which cities are aligned with which sides in the civil war? I believe Windhelm is under the control of the Stormcloaks and the Imperials at least have a presence in southern Whiterun Hold from the introductory scene.

But on the other hand, I believe the Jarl of Whiterun says that Whiterun is on "Whiterun's side", suggesting neutrality, though I could be wrong on this point. There is also an unmolested Talos worshipper in Whiterun, in defiance of Imperial law.

From a RP perspective, as a Thane of Whiterun, the Dragonborn should be sworn to the service of the Jarl, though I believe no oath is ever sworn. Nonetheless, the Dragonborn should choose the same side in the war as the Jarl or else be an oathbreaker. However if Whiterun is truly neutral, the Dragonborn would probably have some leeway in choosing one side or the other. I would have preferred a choice to accept or refuse to Jarl's offer of a title to prevent the Dragonborn being bound to his service.

Any thoughts? Especially with regards to the other cities.

Solitude has a Legion presence, as does Winterhold, Dawnstar and Falkreath I think. Solitude is the biggest Legion city I guess

I'd much rather be a neutral party, who saves the day, and brings everyone together over a cheerful bonfire.

...but since the ACHIEVEMENT states we have to choose a side...
StormCloaks is for me! (I like the color blue more than red)

the legion has always had some corruption.. but overall.. they've also kept everything in check..so.. for the legion!
oh.. n if you want there to be a TES VI you better be for the legion too!.. who knows what kinda government those scum stormcloaks will install.

Gettysburg is on the side of the Union.

Who has Solitude and how long until I can buy a house there if i'm a Stormcloak? Need a house there to be my 'Fortress of Solitude'. Get it? Yeah...

The Empire seems to be in control of Solitude. If you're a Stormcloak, you might have to wait till you finally attack and capture Solitude, which will happen eventually if you're a Stormcloak, according to the achievements.

Yepp, the Legion HQ seems to be in Solitude

So an unknown automatically means failure of a future TES game? Stormcloaks could possibly forge an impressive Skyrim Empire. Though I am pretty sure the game is gonna end with the Empire regaining control somehow and ousting the Thalmor agents from outlawing worship of Shor/Talos.

If you want a future TES game at all you better be pro-stormcloak as they are the ones trying to keep Talos from becoming erased from the mythic. If that happens Alduin can eat the world and make a new Kalpa or world basically.

What about TES 6: Empires? Have more than one empire vying for dominance and you are the one central figure who tries to bring peace to all.

Argonia(lower Morrowind and Black Marsh),
Aldmeri Dominion(Summerset Isles and parts of or all of Valenwood),
Resdayn/Nordic Empire/whatever it would be called(Skyrim and Dunmer and Orc remnants, [upper Morrowind Solstheim and Skyrim),
Highrock+Hammerfell(they showed signs of cooperation in ousting the Orcs),
Elsweyr(a curve ball of an alliance between rebel Bosmer and Khajiit)
Mede Empire(Cyrodiil and maybe parts of Elsewyr and is allies with the Khajiits)

Personally I would hope for a TES that is in a Kingdom surrounded by other Kingdoms of which they all share a tenuous alliance. Would be fun politicking around.

Isn't TES series about fixing great crisis?

This sounds like a particularly badass alliance. I love Nords and Dunmer so I may be biased though.

mabye the stormcloaks will allow Talos worship and wont be controlled by thalmor like the empire.