Emma Frost needs to come into the series. First as the White Queen of the Hellfire Club, then to spin some of the unnecessary new mutants at the Institute off into their own team. (Can anybody say Generation X????)
Banshee and his daughter Siryn also need to make an appearance on the show as both teacher and student.
Psylocke definitely must join the team to give the arriving Angel a love interest.
Firestar (to deal with Iceman)
Dazzler (Especially if she could resemble a mutant Christina Aguilera)
Sunfire (to get a Japanese mutant hero on the show, plus he was one of the original New X-Men with Wolverine, Storm, Nightcrawler and Banshee.)
Lila Cheney (another musician, possibly resembling Alanis Morrissette)
Mister Sinister (to connect Scott and Jean with him.)
Thunderbird (See Sunfire comment)
Mjinari Munroe (Storm's son from the 90's series, to replace Spyke, and show a true mother-son relationship.)
Marrow (This could still happen if you make her Spyke's older sister or cousin)
In the animated music industry, there are two completely different but important girl groups: The Holograms, who are just Truly Outrageous, and the Misfits, who think their music is better. These are their stories.
Law and Order: Starlight Music. (dong-dong)