Russia
Russia with its vast population and many attractions should certainly be considered as a "promised land" for a further expansion. However, unless successful in acquiring Aer Lingus, Ryanair cannot at present easily grow in non-EU Eastern Europe (and I don't believe Russia and its satellites were going to join open skies any time soon). It doesn't mean FR shouldn't plan to conquer these markets at all, they just have to look for a "backdoor" access.
I can recall reading in this forum about (former or still existent perhaps?) plans to use the airport in eastern Finland as a gate to St. Petersburg, Russia (with a bus connection to the town). It would make sense to me. Using the airport located 10 km or so from the EU-Russia border pretty close to St. Petersburg would leave all the beaurocracy outside of the carrier's trouble and would make waiting for open skies agreement with Russia unnecessary for them on this route. Otherwise I do not see how FR could enjoy any significant chunk of this big cake (meaning the population of former USSR and its tourist attractions).