I think Ringo singing Who, Stones, etc. covers would just be...I dunno, demeaning in some way. Not because it's not fabulous music, but because...well, it's Ringo, former Beatle. :-/ I don't really know how to explain myself beyond that - it might be funny in some regards, yes, but that's the music that the Beatles music is thought to be on the same level on, and I don't think he should go for something that similar and something that his group was once so competitive with.
Hope that made some sense...!
EDIT: And Ringo as a solo artist has such a different sound than as a Beatle. It just wouldn't work.