Maybe try events again.
A few sotf (or ACE (which btw is a great name)) exclusive skins will probably help.
Add dlc content, for those who already purchased it, or can be bought seperately for sotf (or ACE)
Maybe tease weapons and combat related stuff using it.
Natural hazards, teleporting pads, maybe abandonned tek bases, with lore snippets.
Game mode ideas:
A game mode with tek, where you find element and tek in the abandonned bases.
A quick fight, where you start with nothing and end fighting with bows and slingshots rather than firearms and armies.
A giant maze.
A malfunctioning ARK, adding one effect every 5 minutes, such as tilted gravity, low gravity, extreme cold spots where the water freezes over, extreme hot spot where things catch fire, broken day/night cycle, etc.
The long run, where you play until everybody dies, no arena size change.
These are just hypothetical, and I didn't come up with names for the game modes, but you can try.
Oh and also, maybe add new bosses, or some from ARK mobile.