Both characters CAN support a movie on their own. What makes you believe otherwise? Anyone can have a movie made around some story in their lives. I mean, look at any movie out there now. Most of them are made around characters infinitely less complex / interesting than Gambit or Emma. All you have to do is look at either of them from a perspective that doesn't directly include the X-Men, so the characters in question can take center stage.
Gambit belongs to a thieves guild that is permanently at war with an assassins' guild. He freelances his skills for shody characters like Sinister (and plenty of others) and has a lot of crap happen to him, emotionally and otherwise. During the course of his undoubtedly exciting jobs, he also manages to get morlocks butchered and get himself forced into marrying a daughter of the rival guild. Remember the movie Ronin, with De Niro? Even something like that could be done around Gambit.
Regarding Emma, same principle applies. She's a powerful telepath from the start, who knows prof-x. She's involved with the Hellfire club, its inner circle and all its political plays, and the management of a team of mutants, not unlike the x-men in terms of structure, who act to forward the club's interests. They made movies about Blade. Blade just kills vampires. Emma does/can do a whole lot more.
As long as the movies are well done and portray the characters and their lives in a believable and entrancing way, I'm ok with them doing as many spinoffs as they want.