Monet-For real, for real.
Synch-Because he needs to ride this wave of recent popularity to a more elevated plane of existence.
Sienna Blaze-Is just a great villain.
Phantazia-Running her own team.
Magik-Is making her way to being a hugely popular character. This might put her in a iconic spot.
CeceReyes- It would be good for the show to have someone who can patch em up real quick. Ease the parents minds after a bad ass action scene. Along with Elixir maybe even Healer.
Maggott-Just for the weirdness and comic relief.
Serafina-As someone who joins the X.
Psylocke-For more than one episode.
Lifeguard-Could be interesting to see her story play out instead on a show rather than the comics. Kinda like how Harley Quinn was introduced on the Batman cartoon rather than any of the comics.
Honorable Mentions
Fatale
Nightmare
Elixir
Shadowcat-Didn't realize she was never in the original.
Marrow
Bishop along with Emma, Banshee, Cece and Psylocke. Need to join the staff.
Cannonball
Catseye
Nanny and Orphan Maker.
Vincente is such an obscure character that I never get my hopes up.
Stacy X- Just to poke the wasp nest.
Xorn