To be clear (which I probably wasn't), I'm just talking about the mechanics. While the story was cute and obviously meaningful, I'm not sure a full fledged "I'm having a baby" motif would work. I just liked the idea of multiple characters on the same screen with different capabilities, and while it's certainly not a unique concept you treated it well. I could also see you using the letter collection for either secret bonuses or maybe a way to get extra powers?
Personally I would, but not sure that I alone would be even close to worth it :) I'm sure there were other earlier examples, but I think back to the Genesis game The Lost Vikings, and at least since that game I've loved playing puzzle games with multiple playable characters on screen at the same time (as long as it's not "real time").