Decided to go for a Swap AU. In this, Cole and Geo are the only ones who have swapped places. I would want to see how Geo interacts with the other ninja. I think Geo would look much younger, happier. Sure, ninja life is difficult, but he's not alone, he's not stuck, he can help others. Cole, on the other hand, would look older, be more stressed, might have some early gray hairs. He has never had to deal with his worst fear of being forgotten until he was transported to this land, and he doesn't handle it well. Eventually, the kids and Bonzle do join him, and his life gets a little better, but that's years away for him.
In this AU, Lou is also dead, dying somehow around the time Cole would have been found by Wu. Cole runs, and Wu doesn't end up finding him. Instead, Wu finds someone else that is connected in some way to earth, which is Geo. This would definitely affect canon a lot, but I think it could be cool!