Thanks for writing the System Design in DIV Blog, they help to clarify the thinking process behind the decisions made so far. I would like to stress that I think there is one point that I have mixed feelings about; the level cap (progression).
You point out that the aim of a level cap is for players to feel a sense of completion. In my view, this is not always true for an ARPG. In D2, I only reached level 99 once, but that did not mean I felt no completion on other characters. A Druid of level 70 with Engima - the works - also felt complete to me; as the purpose I built it for was fun PvP and the 29 extra levels added relatively low value in power to it.
You would want players to have a different ‘levelling experience’ after the level cap, because it would capture “fun” of achieving difficult endgame goals and ranks. To me, this statement reeks of design from WoW’s Artifact Power / Azerite Power. This design is meant to keep players engaged for a long period of time, because the marketing system of WoW relies on players buying additional game-subscription time. For Diablo IV, this would not be needed. End-game progression comes from the variety in (rare/difficulty to obtain) items and skill-builds. The danger of a secondary progression system, like the D3 Paragon System, is the risk of power creep if it offers meaningful power; and boredom/uselessness, if it does NOT offer meaningful power. In other words, a secondary progression system has (in my honest opinion) no place in an ARPG.
Thank you for your time! 