I vote for flightsim, too.
But that would only be a 1/10 of what you can make from this engine.
here's what my dream looks like:
it's a military/ strategy/ flight MMO simulator, that includes every aspect into one, single, breathing, living world.
players would be able to choose a (fictional/ non fictional) nation and be able to fight other nations, they are at war with. and I mean, in every detail. as a soldier in the darkest woods, with your friends next to you, with your wingmen next to you in realistic jets, as a cavalry pilot in todays attack helicopters, yes even as a tank driver or as a marine on board of a ship, cruising the oceans.
BUT this wouldn't be it.
I'd even include a simple strategy/ civ-like interface to create contracts between nations. simple economy creates complete new sets of possibilities. it wouldnt make a lot of sense, if the whole world would be at war at the same time (well maybe for some time). but I'd add simple strategy/ economy features - like resources, diplomacy etc - to create a more complex aspect to the game, where people declare war or support other 'countries' which are in need of help. clans could form easily within the virtual nations and have certain jobs.
moreover, civil flights would be possible, too. not your average tourist-flight, but resource-flights for instance. a simple c130 flight from rammstein/germany to georgia needs protection and instantly, you have 5 people doing the same mission, but coming from a total different approach; one flies the transporter and a 4-ship of f16s escorts it.
I can go on, include real radar and radio control, like it is in current fsx/xplane universe etc etc...
but I fear, this will ever be a dream