I think if Arcane makes a return in a sequel, something I would definitely consider, I would have it act as a dispel for all temporary buffs and a disable for passive buffs for X seconds.
Coupled with that, I think it would also make sense for auras to be always active rather than requiring toggling them each session or after dying. Ideally they would also have some kind of active component associated with them (ex. Pneumatic Burst is always up but activating it gives you the heal).
I disagree with the sentiment regarding sets. There are a ton of examples on this forum alone of non-set builds and builds that use partial sets. Sets are just another piece of the puzzle and offer players a direction to build towards when the absolute openness of the rest of itemization can be downright intimidating to some.
Regarding extra pets, with the way pets are balanced in Grim Dawn, having access to additional pets (aka +100% damage), is simply too powerful of an effect on a single item. The Summon Familiar skill had this on a Necklace and it ended up getting moved to a set because it choked out any variety in that gear slot. It’s not possible to have + 1/3 summon limit, so we opted to put such powerful bonuses on sets.
The same goes for other powerful effects. When we know that certain gear slots are tied up in order to acquire X powerful modifier or skill, we can budget its bonuses accordingly to make something awesome that would otherwise have to be neutered in order to remain competitive with other options in that gear slot.
Taunt is a very powerful mechanic in MP and I would rate it as highly as standard CCs. If pets could hold aggro for the summoner 100% of the time, it would be extremely boring. Likewise if someone builds a super tank and holds aggro 100% of the time, the glass-cannon casters in the party will squeeze out max dps and sacrifice survivability.
That said, Grim Dawn’s threat system is by no means great. We worked with what we had. It was originally even worse because of how it applied threat from debuff effects on every tic, but a full redesign of that system simply fell beyond scope of GD development and wasn’t high-value enough to revisit post-release.
Similarly, the Crowd Control system, or lack thereof, is something I would absolutely want to revisit in a sequel. Something that employs diminishing returns would be much more fun in my opinion but was, as with the threat system, out of scope.
I could write a whole list of things we wish we had more time for of course, haha, but that is the nature of game development.