The King of the hill modes definitely need to be their own thing. They don't fit out of everything else that's in action sack, they ain't bad by any means. But they're pretty counter intuitive for what action sack tries to deliver. Ground Pound Arena, Husky Raid, Riftball are all pretty great and fit action sack's purpose in my opinion. Sumo, Battle Golf, Paintball, Minotaurs and Snowball fight are alright. They get the job done in my opinion and at least feel like fun little modes you would play in customs, although I would turn off team killing in both paintball and battle golf tbh. The Rest of them being Skee Ball, Prop Hunt, Mantis Breakout and I'm pretty sure tank world? Unless it was already removed, but these modes are pretty boring to play through and just serve more as a waste of time than a fun custom based game. I'd remove all these personally, especially skee ball and prop hunt.
If I had to add new modes? My biggest option would be to put in some of the rotational playlist modes, like rock N' rail, mythic shotty snipes, rumble rockets, rocket slayer (the rocket party direct hit rocket launcher mode) (if the king of the hill modes stay) roaming king, mainly those ones would be nice to see in here and would free up some of the playlist space for rotationals. Some FFA Variants would be nice, Super Fiesta FFA, (possibly) Oddball FFA, Vehicle Fiesta FFA, (possibly) SWAT FFA, etc. Gun Game would be an interesting mode to see put into the playlist, I've heard its kind of buggy but from my experience gun game has worked perfectly in customs and I think would be a blast to mess about with against 7 other players. Ro sham bo would be an interesting 4v4 elimination mode to bring to the table for action sack as it isn't like typical elimination since it has the use of random weapons just like fiesta, and my last request would be to do something with the Holiday Fiesta modes like super fiesta joust, low grav super fiesta, super fiesta snipes, explosive super fiesta, covenant super fiesta, All these modes, Could be shoved into action sack not only for 4v4 action but multi team action as well just as a comment suggested, 2v2v2v2, combine that with the 2 king of the hill maps you have in this playlist for use on these modes and other slayer/objective based modes and it would be an absolute blast.
I don't believe all of this would get in here obviously, but if some of this does manage to happen, I will be pleased.