OUAT co-creator Edward Kitsis told TVLine, "Lancelot has a, uh, very 'complicated' relationship with Snow. The two of them have a secret that I'm not sure Charming knows."
So he's her ex? That's where we're all going with this, right? Do you have a different, less obvious direction you have in mind? Hey, everyone has a past! (But does Guinevere know?)
Lancelot will reportedly be in OUAT for at least three episodes, which is what they also said about Sebastian Stan's Mad Hatter. So maybe they've only gotten to that point during filming and they're just not sure who will be needed later. Or is there greater meaning to three episodes?