Thinking about it, I think the mandatory stuff to do is:
1°) Unlock access to the White Witch cabin, access Hiberheim, confront Leandra, then Boreas, then free Icara in order to have her ring and enter in Zandalor house (required to get his amulet and enter the last area).
2°) Collect enough stones to have access to the Trial.
I think the biggest hurdle to free roaming in D:OS is not particulary the overarching quest, but the levelling curve. Unless you are an experienced player, you must do quests in a specific order because enemies having a few more levels than you means you can't hit anything. Therefore, it's the stat system itself more than the quest system that is the most influent, and unless you fix the latter (if you really want an open RPG), fixing the former has little to no effect.
