S.O.S and Powerup Boxes are great! I know a lot of talk is happening regarding rebalancing, but it would be great to have additional points of interest on the map with various challenges. Some ideas:
Airdrop: Takes a long time to capture, but successful defense of this POI gives the player some kind of map-based benefit. A buff. A 5min Gatling gun or Artillery shells wipe the map of enemies.
Rescue: People are trapped in a truck. Defend it for 2min while they crawl out.
Success = player reward. Failure, POI disappears.
Escort: Help a munitions caravan across the map. It's a Trian, it's a Tractor Trailer, its a whatever with a bunch of friendly gunners on it. Huge waves of mobs show up as the vehicles progress across the map. Friendly kills drop the normal drops for the player to consume.
MAP EXIT: (Endless Mode Only)
Current "endless" mode isn't endless. Fix by having a helicopter pad spawn. Defend the pad for 2 minutes or something to exit/save progress. This would allow players to continue on (ENDLESSLY) to fully max out all 3 characters at their own pace and game length.
The framework is provided, leave it up to you guys to figure out appropriate