Well done and good luck, Helen.
You've got a prodigious talent and should do well - especially with footy portraits as there's definitely a demand in the UK and abroad for such things.
As for subjects you could draw... as an Evertonian I could reel off Alex Young, Mike Trebilcock, Brian Labone, Ray Wilson, Tommy Lawton, Dave Hickson and of course, William Ralph Dean (you've already done Bally otherwise he'd be in there too...)
But: if you want to make this a real business; every club has a few iconic players from it's past that people will part with cash to see portrayed.
From the sublime:
Spurs: Danny Blanchflower, Dave Mackay, Hoddle, Villa etc
Arsenal: Brady, George, Henry, Bergkamp, Adams etc
To the ridiculous:
Hull: Chris Chilton (was it Chris...?)
Charlton: Bert Turner, Eddie Firmani, Derek Hales
All clubs have these p;ayers - people will pay for the image, I'm sure of it.
Best of luck with this