While i would LOVE to see addon support in D4 (really think it is one of the features that keeps WoW somewhat fresh), I don't think it is going to happen
I ma personally hoping for some deep endgame features, would it be "raid like" content, crafting, pvp etc. I am hoping for alot of endgame systems, not just rifts and greater rifts - personally a season in D3 lasts about 1-2 weeks for me, then I'm off untill next season. Would be awesome if there would be enought content on release to last about 6-12 months and after that, season content for about 2 months pr. season.