I think the major problem when it comes to establishing an ongoing series for a X-Men character, is the absence of the character in the team book. (and of course sometimes the lack of an supporting cast)
Take the Fantastic Four as an example. Every time one member of the team quits, it would be an opportunity for a ongoing solo series. But even if the solo series would work and sales figures would be ok everyone would know that it's only temporary, because the much larger fanbase of the F4-team book is already waiting for characters return. (And YES I know that Ben Grimm had his own series in the 80s)
Knowing that I would say the essential thing for most solo books would be to focus on the characters life while still being in the Team. Otherwise it would be like the death of a flagship character like Batman. Everyone would know that the resurrection is soon to happen. Batman, Superman, Wonder Woman and GL all have their own lifes and titels besides the JLA, so the X-Men member also needs to have a life apart from the team, because you can't just show the character running around the institute. So the solo character needs to have some special characteristics or abilities. Clark and Bruce of course have some liberties that many minor characters don't have. Flash and Superman have the abbility to pop in and out whenever they like and I think that is a great advange for a multiple series character. Therefore maybe Nightcrawler would be a fitting choice??
But of course there still is the problem with the supporting cast. Personaly I hate it when a character gets his own Mini-series and then the writers create tons of new supporting characters for that series. It's always like "I followed that guys adventures for decades now, so where do all this new guys suddenly come from?!?" Wolverine of course never had that problem. And that's not because of his great backstory. It's because of his whole Lone Wolf attitude. For an ongoing series of a more social X-men member I think only cameo appearances of other members would bridge the lack of an own supporting cast at least at the beginning.