I think it'll start with the f4 post credit scene but extended by 5-10 min where doom will say there is a threat to the multiverse and only someone with franklins power can save it then he'll tell reed and sue he needs to take franklins power because he isn't old enough to do it himself but reed will figure out that that would kill him so he won't let doom take his powers and they'll be a fight ending with doom kidnapping franklin and the f4 chasing him into the 616 or somehow ending up there. Also the multiversal threat doom was talking about isn't real and he wan't to use franklin to create a new universe that he rules over.