One big, easy change I just don't understand is to make use of the campaign... Give the campaign decent scaling and loot that reflects it.
This is one thing almost every ARPG these days lacks, and that is varied content. If everyone only farms 1 way, there's only 1 build. Look at D2:
Countess farming Build, Meph build, Andy build, mlvl 86 build, chaos farming, baal runner, Key farmer, Uber runner all of which was viable for it's own unique reasons.
Not to mention the different brackets of gear going from no gear, to Vmagi to full tals to runewords to being decked out in bloody ebotdz and all that goodness... Also having the super-rare top tier items that let you do crazy things like bramble and what not letting take say Psn Nova at first a build that doesn't work is now one shotting entire screens.
Compare this to Diablo 3, where you get your DB build which is just a gotta go fast buiid. Honestly every classes varient of this feels the same, and then your pushing build. You can't balance abilities to create varied builds, there will always be 1 build that's better but if you have varied content that is what creates varied builds.
Reuse the Campaign in some way it's a huge amount of unused content, maybe let it scale like GRs do with similar rewards? I loved a lot of the boss battles and what not and wish there was a challenging, rewarding version of it.