What does "I will be back" mean in Shingeki no Bahamut? At the end of the series, one of the characters said "I will be back," then the credits started.  Was that just a cool way to end the show or was that saying that there might/will be another season?
There is no green-lit at the moment for a sequel of Shingeki no Bahamut: Genesis. It's based on a card battle game for iOS and Android, so there's an endless stretch of coverage to adapt here.

The best we could assume is that it's "prophetic" for anything Shingeki no Bahamut-related since it is peculiar enough there are no explicit green-lit announcements yet. Anime usually do make the green-lit announcement after the series ended with a hint for a sequel. I take it they meant for something coming, but that's the best everyone (I've searched around) can count on. It could be a sequel, a spin-off (since it's wrapped up), movie, or OVA.

I'm confident, however, that in no way would they be making a senseless "I'll be back" hint without actually following up anything. Not once, in the hundreds of anime I've watched, would that happen. They could, however, announce a termination of whatever project they originally intended to continue the Bahamut hype.