Do you have to play a cannon?  Nope not at all.  Do you want to play a cannon?  Well, if it's available you can apply for it.  Just send us your version of what you think the C is like and a solo of you playing that C and we'll go from there.

Logan/Wolverine - Taken
Scott Summers/Cyclops - Taken
Marie/Rouge - Taken
Jean Grey/Phoenix - Taken
Raven Darkholme/Mystic - Taken
Bobby Drake/Iceman - Taken
Ororo Monroe/Storm - Taken
Remy Lebeau/Gambit - Taken
Jubilee - Taken
The Hulk - Taken

Magneto - Available
Professor Xavier - Available
Kitty Pride - Available
Sabertooth - Available
Nightcrawler - Available
Toad - Available
Beast - Available
And many others not on the list.