Hard to just pick one thing ... but
I would have to say the general game structure ... make the game more focused on something else besides killing enemies and taking flags.
Try something new, like DC Weapons Bunkers, or Battle of Britain, except have that WITH the flags.
For example, if you are attacking a power station, how about have some actual objectives? Bomb these buildings, plant C4 on these generators (in a basement of course), make it interesting.
I think the BF series is a good idea, its just not fleshed out enough, sure it's fun to spawn at a CP and kill, kill, kill until the next CP for a while, but eventually I'd like something to actually
do.
Imagine a battle like Gulf of Oman, first, you start off on the carrier, except, you can't take the flags on the beach until the enemy SAM sites are down and the beach defenses are destroyed. You can kill these any way your commander sees fit; air strikes, spec ops teams, all out assualt, whatever, but in order to advance you MUST complete these objectives, then the flags are open for a fight. You proceed on to an urban setting where its a sector control war, and you have to take enemy CPs and find their "Forward HQ" or whatever, then you can make a final push to your primary objective, the airstrip.
Finally, you could "link" maps together, ie: you play a Gulf of Oman-style map, if US gets the airstrip, you play this type of map, if US fail their objective you play this type. Make a semi-dynamic campaign (choosing the maps would be done by a live admin or a clever script) so after a 4 hour gaming session it feels like you
did something.
That's the kind of gameplay I think would be interesting ...
Perhaps a little to lofty of a goal for a mere mod ... but you never know they might surprise us
