I don't play too many co-op missions either, but to be frank I think we should aim for something that fits ArmA in particular, rather than getting stuck on the old stock gameplay from the era of Quake (Deathmatch, CTF, TDM etc). However those are PvP game modes, but there are still some ideas to be taken from there.
As to Coop games: Even though everyone likes to complain about it, it's the perfect example of a "mode" designed specifically for ArmA: it supports JIP (persistent scores), it takes place over the whole island, it requires a certain amount of coop (different "classes" - Engineer, Medic, driver, pilot, spotter etc) that rely on the different vehicles available in game. It also has a bunch of "side missions" that allows solo'ing, as well as dynamic grouping that allows you to team up and share points with people you like to work with. However, this kind of giant persistant game thing is perhaps a bit too much work for most people, and it also does lean very heavily on the traditional "attack town - take town - repeat" thing. As much as an Evolution-buster would be nice, I think it has to come from the more player-versus-player oriented side of things.
That in mind, what I'd like to see are more "PvP with AI" situations: CTI (Capture the Island) is sort of such a situation, however the massive scale is a bit off-putting, since the AI goes completely idiot as soon as there's not enough processing power and too much going on at once, and since most CTIs just put the AI under the player's command, moving massive groups of them half way across the island is an exercise in futility. So perhaps in a smaller scale?
There were a couple of missions like this that came with OFP's Tonal islands: they were basically coop missions, but there were slots reserved for the opposing team as well: the main group of players were all on one team, trying to defeat waves of AI soldiers and attain their objective (no respawn) while select players led AI squads on the other team trying to stop the player-only side. These respawned. With Join In Progress, some kind of "round" respawn system, this could actually be a lot of fun. However a larger more outdrawn version could also work.
Example 1 (short, round-based game):
BLUFOR (all players) is trying to capture objective X within 10 minutes.
OPFOR (randomly picked X% of all players at start of round) is trying to prevent it.
BLUFOR has no respawn, a lot of vehicles and other assets at their disposal, and a very strict anti-TK script in place. They might have one guy that's chosen as commander or somesuch, depends on the situation/scripting involved. When dead, BLUFOR respawn in a "pen" someplace which launches a spectator script/something like that.
OPFOR have endless (but slow) respawn, are either lone-wolf snipers or squad leaders, and try their best at defeating the BLUFOR before they achieve their objectives.
The round ends when either all BLUFOR are dead, the time limit is reached, or BLUFOR achieve their objectives.
It is also possible to play this entirely without OPFOR troops (=as pure coop).
Example 2 (long game)
BLUFOR has one or many objectives to achieve, and limited respawn: maybe an Insurgency-like system where completing objectives in order (A, B, C etc) gives BLUFOR more "reinforcement waves" or somesuch. Potentially, once all BLUFOR troops are killed, all objectives revert back to OPFOR and everyone respawns at base.
OPFOR has the same role as in the short game: squad leaders for AI or lone-wolf snipers. They respawn at whatever objective is the next to be captured.
Game ends when BLUFOR complete all their objectives/when BLUFOR runs out of reinforcement waves (depending on if things return to the start or not then : maybe they revert to the start, and random players are selected to be OPFOR again).
Could also be played as a pure coop, depending on.
Just talkin'.
Wolfrug out.