STN has potential, but to what extent is FR's scale at STN a deterrent to airlines starting up services?
I'd say it's more of an issue to the airport than to the airlines who use it, hence why MAG will push to diversify Stansted's mix of traffic, although the scale of Ryanair will certainly be a deterrent to some airlines.
The reason why it may not be so off putting to some airlines is as you say, they may not be going head to head with Ryanair. Since Ryanair often flies to smaller secondary airports and small towns and cities (some of which many would probably not even have heard of) which are soley served by Ryanair from the UK market, that will somewhat reduce the competition on the bigger European cities/airports that European carriers that could serve Stansted are likely to originate from.
There is also the same scenario with EasyJet and Gatwick although the difference is that they only account for about 35 or 40% of Gatwick's total, as opposed to Ryanair having 70% of Stansted's. Having said that, EasyJet serve the large European airport/cities of which many routes are flown by either another Gatwick based carrier eg. BA, Monarch and Thomson or a European carrier such as Vueling or Norwegian. In many cases if not the majority, there's direct competition from at least one other airline on EasyJet's Gatwick routes, unlike many of the Ryanair routes from Stansted. Take Barcelona as an example. BA, EasyJet, Monarch, Norwegian and Vueling all serve the route from Gatwick with close to 15 departures a day in the summer months, compared to 2 Ryanair departures from Stansted. I think the sheer volume of traffic on the LGW-BCN route would be more of a deterrent than going in direct competition with Ryanair on the 2x daily STN-BCN route.
Therefore I don't think it would be such a big drawback to any European or long-haul airline wanting to serve Stansted as a destination airport, especially if their home market is not served, or indirectly competes with Ryanair, which is most likely going to account for much of the future growth at Stansted anyway. It would be more of an issue for an airline like FlyBe, Jet2 or Monarch if they were to set up a base at the airport.