EXT has a very difficult task in attracting new operators.
Obviously, as the largest customer, Flybe is gonna have some clout and will justifiably hold its ground to protect its business however this is at direct odds to the airport who need more traffic to grow theirs.
They need to attract airlines that will offer additional services to Flybe but that will not damage the existing services as Flybe is their 'bread + butter' daily source of income. Any GVA service by EZY will be flown by their Swiss division as it is in Bournemouth, however I would imagine that within a couple years you may see additional destinations such as Grenoble or some sun routes creeping in.
I think Flybe may need to accept that the days of having airports to themselves is coming to an end. You only have to look at BHX with WW, BHD with FR, and EMA,BRS,LTN,STN,LPL where FR + EZY offer services to see that things are beginning to get tight. At the moment all is well as the economy is bouyant, however if there is a downturn around the corner we could see one huge bloodbath as airlines more actively compete against one another for passengers.
Flybe will be able to survive out of Exeter. The Q400 operating to places such as GLA/EDI compliments the EZY operation out of BRS and does not actively compete with it due to capacities. It is likely that any heavy interest shown by EZY at EXT will only result in them damagin the Flybe operation whilst removing traffic from similar destinations offered by themselves ex BRS.