I've definitely enjoyed the ACT mechanic since it has been implemented, and can't help but think of additional ACTs for more variety. Here are some I've been thinking of. Of course, balance etc. are subject to change, just ideas for how they might work if implemented, and mostly just a 'for fun' kind of suggestion, although still would love to see more ACTs in the future, regardless if any of these make it in or not.

ACT: Berserk
An ACT that decreases cooldown and increases attack speed upon killing a mob (killing blow only perhaps?). Could stack, up to 3 mob kills for max benefit, with the period between kills before expiring between 3 to 5 seconds, with maybe a cooldown before the next usage upon expiration.
ACT: Speed Blitz
An ACT that causes your DART to deal damage on enemies you pass through (damage based on AGI stat somehow?). Would have a considerable cooldown between usage, with higher tiers decreasing the cooldown slightly, but not to the point where it would be infinitely spammable or anything with a high DART stat. Maybe for fun points, could work with the DART stat super jump too (shared cooldown).
ACT: Contagion
An ACT with a unique DoT debuff, Contagion. When equipped, an icon will appear over your player when Contagion is ready (cooldown of 15+ seconds?). Upon attacking with Contagion ready, inflicts the unique debuff on attacked mobs, but also on the player. When the player is debuffed (lasts 3 seconds or so on mobs and players?), all attacks will inflict Contagion on mobs as well. The cooldown begins when the debuff on the player expires. Suggestion subject to heavy change, but the idea of a DoT or debuff based ACT would be cool.

Again, these are pretty much just 'for fun' suggestions, not really intricately balanced per se, but ideas that I think would be cool if added into the game in some shape or form, even if heavily tweaked. A couple other ACT ideas I had which I'm not quite sure how they'd work in-game include a Parry and/or Block ACT, which I'd imagine would have to have some sort of near-perfect timing on player input of some sort.

Ideas for buffs to 2 (in my opinion) slightly underwhelming ACTs:

-Allow First Strike and Fortified ACTs to stack to two (7.5s per individual stack) after reaching Extreme tier and above.

Overall, love the mechanic and I think there's definitely room for more in the future. Do you have any ideas for new ACTs or changes you'd like to see? Feel free to post them here!