Probably, the game was a critical success and I believe it sold well. The problem is they are a small company. Right now they finished Poker Night 2 and are likely working on getting Fables done. Then they might be more likely to work on a Walking Dead sequel or (wishful thinking) another Sam & Max game.

I think the ending was a reference to the finale of the 1st movie, which was just a joke.

Telltale is currently working on Game of Thrones, and Tales From the Borderlands. That said, Back to the Future was their first big success so they might have a deal in the works.

Maybe they meant the story continues in the animated series. :)

It was probably both a reference and a possibility of another series.

Obviously the reference is there and they could keep it at that if they don't decide to carry on with the franchise.
"Hey, it said 'To be Continued' at the end! Where's the next series then?"
"It was just a reference. We never intended to continue with it."

If they do decide to carry on with it, the window is there for it, though I don't know what they could do in terms of story.

It's quite clever. It keeps the fans hyped with no actual promise of more.