I have avoided this section of the forums like that plague for literally years, but I just can't resist. I was just poking at the Random Strip button, and I realized something terrifying:
Dora and Marten have been dating for 1200 strips. Their first date was around #600. That is a GIGANTIC number.
As for what will happen... it's hard to say. Part of me wants to say it's not over, either due to Marten being unwilling to give up or the previously put forth idea of something else (intersvention is the greatest pun ever) influencing Dora. That may, however, simply be me projecting my personal life onto the comic. Judging from Jeph's Twitter and the newspost, he is taking this very, VERY seriously, and it's hard to take that as any other way than a breakup. If that does happen, I can see Dora developing a little more over time and possibly taking Marten back. But if that happens, it's gonna take a LONG time, especially with Jeph's tendency to put minor silliness before plot development in the long run.
As far as in the short term... I really don't see any possible romantic matches for Marten right now. Faye already has some conflict developing in the wings between her relationships with Angus and Sven, and it's pretty clear she's not available for Marten. Hanners is the only real available female character, and despite her recent growth through her date with Sven, I feel like she's just not mature enough romantically to fulfill the role of a romantic interest. If anything, I would suspect Marten would take a stance of "just ended a serious relationship, not looking for anything right now". The only other alternative would be to pull a new female character out of the woodwork, and there are a couple of reasons why Jeph wouldn't want to do that.