If the next DLC were to involve Hammerfell, is it not possible that the provinces of Hammerfell and Skyrim join forces to stop the invasion of the Aldmeri Dominion.

I mean, it makes sense to me. That would be a way to tie Hammerfell/Reguards into the current story of Skyrim.

Anyone else have any ideas on what the DLC may be about? I would love to discuss this.

One would imagine that there will indeed be a DLC that either resolves the current Thalmor - Imperium conflict (unlikely) or furthers the story in some interesting way.
Since Hammerfell has been on the receiving end of Thalmor nastiness and Skyrim has a white hot hatred of the Talos-suppressing Thalmor, as well as the fact that Hammerfell has broken away from the Empire and Skyrim is trying to do so, it would tend to drive these two nations towards each other in a positive way.
However, there is a significant bone of contention between the two:
Dragonstar.

I'm sorry, what now? I thought there was a civil war over it? That would denote that not all in Skyrim want to break away.

The Civil War isn't completely over. Skyrim is in the process of breaking from the Empire. Just because one of the leaders dies, doesn't mean that the war is over.

The only problem that I could see with a DLC where Skyrim and Hammerfell join forces would be if you sided with the Empire instead of the Stormcloaks. They could probably work around that though.

People want that [censored]ing Dominion war so bad, they'll rationalize it into anything.
And I grant you, sir, a soft golf clap for completely shafting the Imperial players with your idea...Very classy...
"I mean...yeah, there might be an issue if you sided with the Empire...I mean, if you wanted to do that...I guess..."

Haha I didn't mean to shaft them. I just thought about it. Maybe the Empire would finally stand up to the Dominion or something?

I never understood why they were so powerful to begin with. It's the entire Empire that spans the majority of the continent and they're scared to fight the Aldmeri.

I love the idea of adding hammerfell to the main story but I'd be scared that it would feel rushed in an add on

They should save the war with the Thalmor in the next game. Wouldn't be surprised if it took place in Hammerfell.

With so few details on what actually happened during the war leading up to Tidus Mede being forced to agree to the White Gold Concordate, more than likely with several sharp objects pointed at him, I think it's safe to assume the Dominion isn't marching with pop funs and sparklers. And the Empire hasn't exactly been the most united thing ever after Tiber Septim passed away.
If you bother to pay attention to most Imperial NPC's, it's obvious the Empire is biding it's time and formulating a plan, one that's more subtle and not as life-costly as the average dunderhead Nordic tactician might develop.