What if they make Sabretooth a recurring character? But that would assume that it gets more than one movie. The first movie and can be an origin movie, revolving around NOLA but by the final act of the movie is in Paris (or at least an equivalent, perhaps NY instead). The Genevieve story might be too convoluted if Belle is going to be present as an interest as well, so I guess the story can be tweaked. Perhaps Bella can accompany him on the heist (by choice or not), gets caught up and takes Genevieve's place as Sabretooth's captive. Now the question is, "who dies?" we follow the story and have Gambit save his brother or Belle? Being the cause or even involved in her death/murder would be enough to get him banished from NOLA imo, and even motivate a "death sentence" for him enough to be pursued by the Assassin's guild or at least a pissed off family member or two...
Assuming it gets a second movie, we could pick there, Gambit on the road alone and full brooding mode taking whatever job comes his way, powers flipping out all the way. He ends up taking up jobs he shouldn't and comes across a guy (Sinister) that can help him solve his issue and in return wants a errand run which does or doesnt lead to the Morlock Massacre and involves the Marauders or just Sabretooth. The final act would be him trying to stop what he initiated single handedly, which can undoubtedly very bloody and violent.
Yeah I guess it depends on how quickly they progress through his origins. I figured the first hour to hour and 15 minutes would be the Thieves and Assassin's Guild and would end with the wedding and exile. Not sure it makes sense to show that for the whole movie.
Then the next hour to hour and 15 would be his wandering and encountering Genevieve and Sabretooth. I would probably depart from the comics a bit and have all three being a part of the Heist team maybe with Scalphunter, Claire, and a couple of non-mutants. At this point they mutants in the group are hiding their powers from each other unaware that others are mutants as well. Gambit is the playboy of the group while Sabretooth secretly loves Genevieve. Long story short a heist goes wrong and Gambit has to choose between saving Sabretooth and Genevieve and chooses Sabretooth because trying to save Genevieve would have been riskier and more dangerous with Sabretooth. So he plays the odds in trying to save Sabretooth first. At that point Sabretooth ends up revealing his mutant powers letting Gambit know he should have gone after Genevieve instead of him. They rush to save Genevieve but arrive too late resulting Sabretooth attacking Gambit revealing Gambit's mutant powers and setting up their rivalry. That could be the climatic battle as those two battle it out with Gambit eventually winning but still feeling incredibly guilty.
That could then set the stage for the second movie in which Gambit is a loner now and just does jobs on his own with his powers starting to flare out of control. That is how he ends up with Sinister and in exchange for Sinister's help, Sinister asks him to lead a heist team to steal something of importance from some bad people. This team includes Scalphunter and Sabretooth but Gambit still feeling guilty over Genevieve and indebted to Sinister decides to agree to this request and hence that is how he ends up aiding the Marauders only later finding out those alleged bad people were the Morlocks and then the big battle is him taking them all on to no avail. The movie can end with him wounded and saving Marrow.