Well, I made a list.
Done:
AoA Rogue
AoA Jean
Ultimate Magneto (No Helmet)
AoA Toad
Ultimate Toad (Blue Outfit)
Ultimate Toad (Brown Outfit) <- not sure if this one was ever in comics
Ultimate Sabretooth
Classic Sabretooth
Modern Pyro
Modern Sunfire <- once again, not sure
Modern (Black/Dark Blue) Juggernaut
Ultimate Rogue
Apocalypse (Blue/Gray Outfit)
Ultimate Beast
AoA Sabretooth
AoA Bishop
AoA Colossus
AoA Cyclops
AoA Gambit
AoA Nightcrawler
AoA Storm
AoA Wolverine (Weapon X, not the AoA exclusive Wolverine)
Original Cyclops
Modern Gambit
Modern (Black Suit) Mystique
Green Phoenix Jean
Black Trenchcoat/Green and Yellow Suit Rogue
Classic Juggernaut
Classic Sunfire
AoA Beast (Dark Beast)
Astonishing Cyclops
Classic (Blue with Yellow trunks) Cyclops
Classic Nightcrawler
Ultimate Nightcrawler (With Swords) (HUD already done!)
Uncanny Nightcrawer
AoA Deadpool (HUD Head already done! YAY!)
Ultimate Iceman (Human Form)
Original Jean
Ultimate Jean
Doing/Will Do:
Archangel (Pink/Blue Bodysuit)
Modern Angel <-again, unsure if this comes from comics
Modern Banshee
Original Beast
Uncanny (Bald) Bishop
The Other Bishop (I really have no idea how to describe it)
Blink
Ultimate Blob
Dark Blob (from XML1/included in XML2)
Ultimate Colossus (Human Form)
Destiny (old woman, no costume)
Modern Forge
Ultimate Gambit
Modern Havok
AoA Juggernaut
Classic Juggernaut (no helmet)
Classic Lady Deathstrike
AoA Magneto
Ultimate Magneto
Classic Mr Sinister
Omega Red
Modern Polaris
Dark Pyro
Modern Quicksilver
Sauron
Classic Scarlet Witch
Modern Scarlet Witch <-not sure if this is from comics
Astonishing Kitty
Stepford Cuckoo
Ultimate Storm (Spiky Hair)
Stryfe
AoA Sunfire
Modern Emma
Weapon X (Nekkid) Wolverine
Zealot
AoA Mikhail
AoA Abyss
AoA Holocaust
Modern Grizzly
Guardian
Those are all the skins I plan on eventually doing before I call operation 'Convert XML2 Skins' complete. I know it is one buttload of skins, but that is how many there are to do

EDIT: I'll be using this post to keep track of my progress.