Just make the game very modable and open, with a good script language like UT3, and plenty of documentation like UnrealWiki. While I thought the graphics were not that great for Arma, the ones in Arma II look a lot better and more realistic. I play the UT mod Infiltration and as coders and artists have moved on to other projects, the community is looking for a new fps to play. We are hard core realists so I hope that we will see weapons like the PKM machine gun, FN P90, modern variants of the RPG, etc.
One thing I would like to point out, is that most FPS these days do not make realistic vehicles. At least for jet aircraft, I suggest you take a look at a very real simulation like Open Falcon, a mod for Falcon IV. This way you could bring the flight simulation community in as well. And we could have one awesome war.
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