This one should be a no-brainer. Spider-Man is one of the most popular comic book characters in history, and his movies have made billions of dollars over the last decade: what's not to like? As I said, I expect we won't ever see DC and Marvel characters squaring off in a Lego movie because of thorny rights issues (and I hope I'm wrong), but Spider-Man would fit perfectly into the world established in the first movie. Seeing his Lego web-shooters flying around, swinging from building to building, would no doubt look hilarious, and I'm sure Lord and Miller could crack a few meta jokes about the recent reboot that nobody wanted. And of course, we could see a little Peter Parker too, just as Bruce Wayne had a small appearance in the first film. Maybe even give him two heads to switch between: one which looks vaguely like Tobey Maguire, and one which looks like Andrew Garfield.