I was reading the comments and when I read Terranisaur comment I thought popped in my head.
First I would like to say that the primary games that I play are - D1 Thank you GoG), D2, D3, PoE, WoW, Divinity, Pillars of Eternity etc.
I really like that some characters as the Sentinel have good utility/support aspect within their passive trees, which can be a huge benefit to group play and I also believe that mixing ARPGs mechanics with MMO mechanics is not a good idea, however have you considered to have an alternative Spec for each character?
Something like the Armory in D3 where you can change your gear and skills with a click of a button that will allow you to jump from one build to another, but not more. This will allow a player to have one Solo/DPS orientated build and another utility build.
P.S. I really haven’t put much thinking on this, just thought of it while I was reading.