Well, Sally does make an appearance in Sonic Spinball... for a second every bonus stage if you free her from a roboticizer. So, she actually is in a Sonic game already. Tee hee.
yes, it would be nice to put Sally in Sonic The Hedgehog, Sonic Adventure DX, Sonic Adventure 2, Sonic Riders, Sonic Generations, Sonic Colours, Sonic Black Knight, and in all videogames