Spirit airlines in the USA have had a similar policy for some time so i wouldn't rule it out. Ryanair have to look for additional sources of revenue as all airlines do.
The credit/debit charge ruling will simply be made a admin booking fee for using the system not for payment type so thats the way round that potential loss of income. make it "Free" on a limited number of flights and its optional so therefore they will avoid having to show it in headline price... naughty, irritating and starting to backfire but perfectly legal.
I'm sure they will increase hold luggage charges, check in fee, etc. The often floated idea of a charge to use the loo is just a
PR stunt and wont happen but perhaps they will increase seat allocation fees. Not a bad idea for a family wanting to sit together and good source of revenue. Beyond that I'm struggling to thinks of new ideas and that's when the business model will evolve. As others have said if Ryanair became customer friendly or encouraged interlining by saying providing you leave say 2 hours between flights etc its Ok or if we cancel one leg you may cancel the other they would be on a winner. Ryanair say they only cancelled 10 flights this summer. That's a great record but if you asked the public they probably thinks its significantly higher and they fear is being stranded by Ryanair late at night with no help, transportation or anything and told "f@@k off you paid peanuts what do you expect"