Wow!  Absolutely my favorite of all of these.  Double love for the Reed wrapped around Invisi-Sue.  Also avoids the issue of whether to give her a 1961-style haircut!

This is set in the wreckage of the original space ship, right?

I'm going to suggest something along the lines of the original Contest of Champions wherein some Elders like the Gamesmaster (is that the name of the Elder who has a gambling problem - I forget) challenges someone like Kang or Immortus or a new Elder of the Universe to a bet as to who (which FF) would win in a duel.  Meanwhile, Kang or Immortus has some ulterior motive to get one or both sets of FF to obtain some item or mission to further his devious plans.  Gamesmaster gives the other FF a motive to stop the other FF from accomplishing their goal (stopping Doom from some diabolical plot or the Earth from some catastrophy).

It's down and dirty until the two Reeds figure out what's going on and end up frustrating the Elders' plans.  Maybe the Reeds even switch places unbeknownst to everyone as older Reed shows younger Reed how to change his face to appear different.

Moment of emotion is the reveal young Susan gets when she finds out she ends up marrying Reed and has children with him. Maybe make it more pertinent in that young Susan is dating or considering dating Namor and she uses this to break if off with Namor.