I seem to recall that Brymon did a proving flight with the Dash 7 into ISC when they were phasing out the Twin Otters (ha - G-BIHO still going with Skybus!) and the proving flight was sufficiently "interesting" to see the whole project abandoned. I can't think that, with all of the changes to EU-Ops and performance criteria which have taken place between then and now, the situation will have improved in any way.
And in any event, planning the future for the next 20-30 years around an aircraft type which is already obsolete is not a particularly good idea. Best bet is to get the runway to a slightly improved standard to be able to support operations such as a Dornier 228 and accept that any larger aircraft will mandate increases in fire cover, ATC, introduction of EC300-compliant security screening for pax and all sorts of other expensive infrastructure that the passenger throughput simply isn't high enough to generate revenue to pay for.