Ryanair may seem bigger that Easyjet at MAN, and that's because Ryanair use far more 'away based aircraft' than Easyjet, but, Easyjet have more based aircraft than Ryanair (easy has 11 vs Ryanair's 8 (9 in peak summer).
In terms of potential routes from Easyjet:
Rome. Maybe, but not a priority as we have Ryanair, Jet2 and Vueling on the route.
Toulouse: only against a 2 weekly Jet2, so possible
Faro: big missing route in my opinion for Easyjet. Portugal holidays on the up, so, scope.
Lanzarote: maybe in time. Again, surprising Easyjet on,y have one canaries route from MAN (which is TFS)
Dubrovnik: less likely now they are serving Tivat, and of course already serve split.
The ones to watch for big expansion are Norwegian.
Likely to have 8 routes this winter (the current 6 plus Tenerife and Gran Canaria)
Palma is to be added from next May:
D85891 PMI 0940 MAN 1130
D85892 MAN 1215 PMI 1550
The/Thu/Fri/Sun
Should be bookable later in the year.
And of course, the growing rumour of a base.