I don't think Ryanair would buy another type of a/c, you would totally have to change the whole set-up of Ryanair, Crew would need re-training, Engineers re-training, manuals .........
The biggest cost of a route is the pilots wages, so even if you had a smaller a/c like a 737-500 you will still be paying the same for the flight deck, and only save alittle on the cabin crew, and fuel, but saying that the 737-800 is great on fuel.
Loads more routes for it to move into yet before we can think about smaller routes like STN-BLK. I wonder when we will see flights to eastern europe and greece.
Lee