For flybe to succeed at stansted they need long haul airlines to move in as well, so they can feed passengers on to the long haul. The flip side of this is, for any long haul to move into stansted, they would want some one like flybe to feed them.
I'm not sure that they do. With the exception really of LGW and MAN and to an extend BHX and GLA, most FlyBe airports in the UK are smaller regional airports with very few if any long haul connections. Some of BE's biggest bases like BHD, EXT and SOU for example are virtually 100% FlyBe airports. Ryanair used to fly STN-BHD up to four times a day so I'm sure BE could fill a few Dash 8's or Embraer aircraft a day.
Stansted has more scheduled European destinations than any other airport in the world. There are many destinations across Europe where their only direct flights to the UK are from Stansted airport. Lets take this for an example:
Someone from Northern Ireland wants to get to one of those less heard of Ryanair destinations eg. Carcassonne, Lublin, Pescara... the list is huge of airports soley or mostly served from Stansted in the UK. Take Carcassonne for example. I bet that someone wanting to go there from Northern Ireland maybe for VFR (visiting friends & relatives) or for lesiure purposes would find it buch better to fly to Stansted, then book another flight to direct to Carcassonne rather than fly direct from Belfast to somewhere like Nice and then have to travel a long way at the foriegn end.
I know that sounds quite a niche market, but because the list of short-haul destinations that are unique or mostly served from Stansted is so huge, it soon adds up and to be fair, most of their customers are going to be flying to go to London and Stansted is a good gateway for them. And, FlyBe, which is now almost starting to suffer from the competition at Gatwick by BA, EI and EZY primarily, could bring a a range of domestic services that people of North and East London, East Anglia and into the Midlands would highly appreciate not to mention all of the people living near the new destinations that would be bought to Stansted who would in a lot of cases have more choice of London airports to fly to.