Im suprised Ryanair has not gone down the Canadian affairs line, They from what I can make out allow you to pay to pick any seat you wish and if you dont pay to pick your seat then you get whats left ...
Ryanair could lower the advertised fare by a few £ or Euro and put a price on every seat, lets say £10 exit/leg room, £5 window, £3 isle and £2 middle, This would mean putting a seat plan on the web/booking page but I think it would be a good optional charge earner.
Also this would work well with passengers traveling in groups or with children, I think many are put off flying with Ryanair to holiday destinations as there is no real way of guaranteeing sitting together
Boarding would be in three parts
1. Infants/special assistance.
2. Seated passengers.
3. All others.