Disclaimer: Skip if lazy to read
First of all, I have no idea if anyone suggested this before and let me tell you what I know about Grim Dawn and how far I played through the game.
I pretty much know nothing of storyline as I skipped all dialogues and not yet start Act 2(after killing Warden?). I have spent almost all my time so far understading the the skill/class system, item, stats and Act 1.
*So, below suggestion is base on what I have played so far.
- Multi Character Control/AI Companion:
For me, I like this kind of game, intense class/skill system and item with crazy stats, but one thing, it is so boring when you are playing alone(I don’t play online, sorry about that.), I’d suggest something like Multi Character Control(MCC) and AI System.
Maybe the MCC system can be something like Inotia 3/4(mobile game) or Granado Espada(MMORPG) where the other character can be controlled or set automated. In this case, you can support each other and maybe release a stronger boss where party syncing is needed. I see that the builds suggested here are mainly for dealing damage, or at least more than 50% of skill setup are to deal damage. However, what if you can control another character(or automated) for this character to be be made a full tanker/supporter? That’d be fun with stronger bosses adjustment. And for myself, I love to play full tanker and supporter 
Maybe, to introduce the companion system, there are quests that you have to go through to get skill/stat points for them.
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If by any chance, this is implemented, supporter class like Priest is needed, as currently we have too many debuff skills but not really much skill that buff pet/myself except equipment. Also, with a role of tanker available, it is also possible to make Soldier-shield + Priest class. It’d be fun to play that. And some of the current skills need to be adjusted if Priest is available, else you would be invincible
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Lastly, I’d like to suggest some advance skill for pets. Knowing that pet has their own skill, but all of them are casted automatically. If there is some advanced skill in Shaman that can control the timing for pet to cast skill, that’d be best.
Or maybe a total new class that totally contributing in pet skill tree where even the pets can buff themselves.