Bringing back and polishing Anarchy would be one way.
I vaguely feel the maps currently in the game are also the perfect size for an extraction shooter style gamemode, if we get rid of the cars or turn them all into wrecks and decrease the amount of players per instance.
From a creative standpoint, you could chain the 2 gamemodes up by lore.
In any case, a lot can be tried and tested to get some population, but in the end nobody is going to stick if the core gameplay is miserable. The shooting in this game still feels really good, but because every other core aspect of this game is just utterly miserable, I have no reason to actively play.
I'll come out of my retirement home when the game is fixed in that regard.