Fiirst of all - i really love the game. There are issues right now but i'm sure that with time and effort you'll manage to solve them and i know already the main focus is the servers and online experience (which is why i'm playing offline for now).

Anyway. It appears that when joining another player your character story progression is being overwritten by the one of the leader. I made a lvl1 character and joined a friend that was at act1 last boss. Unfortunately leaving the group and trying to play alone i have all the act1 unlocked and i'm at the quest for the last boss, with no means to reset the story.

I don't know if it's a bug or a design issue; i know also that there are workarounds for offline characters by editing saves, but obviously not usable for online characters.

Please take a look at this when you have the time. It would be nice to have the chance to replay the story with a fully decked lvl90 character or basically at will.