I think it would be fun to have an “Onslaught Survival Game Mode”, selectable from the Main Menu. The Devs would not have to create any new assets I would not think.
The player would click the game mode and start with a fair number of landmines, grenades, LP gas cans, some good guns and ammo…ect. 
The start location would be an area to hold & defend ( something like The Castle ).
With something like 5 minutes to go out and set-up traps to get ready, Wave One would start and have a fair number of incoming low-end bots that would come in and attack.
Once Wave One was over then the player would run around, pick up more supplies from ammo crates nearby. Then 5 more minutes to re-set some more traps, and then Wave Two would start with higher level Bots attacking! 
and so on and so forth for as long as they can!
This could be played Solo, or Multi-player with more bots per number of multiplayer friends!
This would be a blast I think! 