My original plan was for him to not really show up again until after the battle. However, I feel like that might be a bit anti-climatic or make it feel like too much of an afterthought. So here are the ideas I have considered and would like your guys input on them.
Original Idea: Marc would not show up in the final battle. However, afterwards they ask Tiki (who in FP has taken over Naga's mattle) to teleport them to where Marc is. They find him in a graveyard, near his mother's grave (potentionally about to commit suicide). He spots them, tried to run away, and even pulls a sword, but trips and the weapon is easily taken away (he doesn't even struggle). Lucina ends up pulling his hood down, revealing his appearence and that he is crying. He ends of breaking down and big family hug.
Idea 2: Since Morgan doesn;t have much to do in this final battle anyways, she ends up engaging in combat with Marc again. Unlike before, she doesn't activly try to win, meaning Marc starts to win this time. Instead Morgan tries to convince him to stop fighting. This begins to work, but then everyone gets teleported out of the timeline like normal (unlike the game, Morgan, Lucina, and Robin do go back to the FP timeline to help again in the final moments). FP!Lucina tries to do the awakening, but finds out the naga of that time is dead. Marc actually steps in then and heel turns, but is about to be killed when Robin, Lucina, and Morgan reappear along with the newly godified FP!Tiki.
Idea 3: Same as Idea 2, only with Lucina being the one to have the fight with Marc. However, this would require me to make more changes since....
...my plan was to have Lucina and FP!Lucina run into FP!Grima, get knocked away fairly easily. Then Robin ends up bursting in, puts up a hell of a fight, but still gets battered around at the end once its clear that 1)Grima is stronger in this timeline, 2) Robin's magic is weaker now that he no longer has the heart of grima, 3) actually doesn't pose a threat to Grima, since he can't kill him perminently anymore. Badass line from Robin "Yep, can't do anything thing to hurt you. I pose absolutely zero threat. But to be fair, I am a really good distraction" into Chrom coming up from behind and just wailing on Grima with fully awakened Falchion.