I would pay 1 USD per month easily but honestly I don´t wont infinte changes to this game. I just want to see this happen:
Season 6 - Calling to Improve Itemization
And some QoL features like a very basic basic lootfilter (no lower pots, scrools or reduced font size etc.), stackables (up to 10x or 99x for gems, runes, keys). Maybe adding the three missing quests in act 4 and relocate Cain in act 5.
Some class balance changes as proposed here:
Season 6 - Proposed Class Balance Changes
And some love for Act1 and especially Act 3 mercs.
Season 6 - Few Merc Improvements
That would be emough for 3-4 skilled devs over 12 months I would assume. Therfore I would be happily pay 5 USD per every additional char slot every season!
If combined with the need to complete every season with a new char (hell) in order to be able to purchase 1-2 new slot every season would even help to fill the servers a bit as well!