So with all of the hubbub about people wanting/not wanting grp play, I noodled a bit on what some of the really great direct benefits of grouping has been in some other games I have played. One of the best things I’ve ever seen in a game that really made it beneficial to group was classes that could debuff a target to make it weaker for an other class’s dmg.
Not sure how practical that would be for D4 as it seems like all classes basically have the same dmg types. In other games I’ve seen things like
Class A → Deals Fire dmg debuffs for spirit dmg
Class B → Deals Spirit dmg debuffs for Ice dmg
Class C → Deals Ice dmg debuffs for Fire dmg
Anyway, probably a terri-bad idea and would never work in D4 but if it could, it might give people who are not interested in grouping a direct tangible reason to do so.
Good news! Diablo 4 has that.
Can your class apply Vulnerable at all times?
How about Curses?
And Slow?
Frozen?
Knockdown/Stun?
(No one class does all of that)
What D4 doesn’t have is a buff/debuff bot class. And that’s a GOOD THING. It sounds like you don’t want an actual player to party with, but someone to follow you around to buff/debuff.
Good news though! Mercenaries are coming!
Gotcha! I’ve only really played sorc but noticed things like Frostburn were not class specific so assumed all classes had a way to deal fire/cold dmg. I guess I just need to play other classes to understand what synergies are available. From your post I’m guessing it’s there but in a more nuanced way than just dealing/debuffing for different dmg types.
And no I’m not looking for a debuff bot. The game I am mentioning was highly Grp v Grp pvp oriented. no room in that scenario for a buff/debuff bot, but when you had all 3 main dps assisting on the same target and debuffing for each other it would disintegrate fast 
AFAIK sorc doesn’t have curses but they can do all the rest (well not knockdown but stun)