Hmm. I can see Min-Gi and Ryan having some problems, mainly because the other issue of the way Ryan bosses him around has never been resolved. Ryan would totally let the rock star life go to his head and be involved in drugs, partying etc, Min-Gi would be the more practical. I know there is a theory that he returns as an old man after losing Ryan but I don't know about that.
Jesse has the happiest ending. His life would improve from learning to cut out his bad friends and he would eventually marry Lake after college, and settle down in a nice rural area to avoid nosy reporters asking about a metal gf.
Alrick lives in Amelia's heart. Simon might be a denizen according to some theories that if you die on the train it gets to keep your soul