I see this as a "kill room" challenge (from Armory) type mode, but without having to unlock things first, showing first hand various aspects of vehicles, team and tactical movement and play, and of course, being multi-player, not single-player. Obviously, it couldn't really incorporate jets and such, because 1/50th of the map would be damn small. So maybe a town, and highlight tacticals and common vehicles (cars/trucks/Ural/whatever), along wiith APC, maybe a tank, a couple HWMMV variants, or equivalent, for each side. This would also give players the chance to really see the dynamics of gun-play first hand, and in a CQB environment, which in ArmA2, CQB is already quite brutal. Death happens quickly when you act without thought. The projectile physics at range helps people learn more of the same elements that are in BF3, such as bullet drop and time to target.
So really, instead of 'kill room' maybe it's 'kill box' instead. Cordon off a 1 klik by 1 klik area around a small village, spawning at a F.O.B. on some random (but opposite side) area outside of town, on the edge of the kill box, and it runs till a. all opposing team members are killed or b. mission objectives are complete, or c. both, or d. all 'tickets' (to borrow a term from BF3) are depleted. No timer on the match, no points to be gained, simply complete the mission objectives and/or kill the enemy entirely. The game mode would still incorporate all the main features that makes the ArmA3 engine so fantastic and attractive to 'serious' (non-casual) players. To be clear, I'm speaking about casual in the sense that ArmA2 can be picked up and played casually, but you don't get very far, it takes a bit of dedication to really get into the meat of it. No offense to 'pick-up-and-play' gamers at all, I like those games too, but that's why I also own several C.O.D. titles and several Battlefield titles.
People would learn movement dynamics, and become familiar with the way the game operates in PvP in small environments which incorporate both mid-long range gun-play and still be able to get into those areas that require snappy, fluid movement and aiming dynamics in a CQB sense, and learn the difference between concealment, cover, and out in the open combat versus urban combat, etc. Also, how the vehicles come into play, what their vulnerabilities are and such. I can see this mode being immensely popular similar to BF3, if by design it was like a 3 segment sort of domination match as well. Finish one killbox, and open another adjoining area to move into and take, or fail at taking, whichever is the case. This would make it like a rush match, sort of, except it could easily be defined much differently with various objectives to complete, introducing the complexity of mission types, how to read your map, navigate a region, find the objective, etc. It could be a great mode.
But that said, the map has to be big enough to cater to all of the play styles. Those that like to sit at range, and pick people off (recon, sniper, marksman, rifleman) and those that are more comfortable with choke points and such. It would require teamwork from all, because each play style bolsters and assists the other.