I was just thinking about this today. We have a hidden to friends option, so why not a hidden to all players option?
I know this might be hard to add into game, but it could also resolve a lot of problems.
1. People could avoid more conflicts this way.
2. Less lag.
3. No complaints about reagent/chest stealing. (Having conflict on this one, either we get to have reagents in this mode or none at all).
4. Less false reports. (Probably not by much.)
5. People wouldn't be able to jump into battles.
However with this, we should not be able to:
1. Switch Realms
2. No friends porting.
3. No porting to other players (unless you want to go out of solo mode).
4. Since you aren't seen to other players, maybe no buying things in the Bazaar that other players have sold? Considering players are not seen or heard from in this mode, it would make enough sense, however this I am still debating.
I'm not sure how being hidden will prevent that. Even if your wizard is hidden/invisible, everyone can still see the battle circle and the enemies you are fighting in the street. However, when they join your battle, they wouldn't know who they were teamed up with or may even think you fled, possibly leaving you with three or more enemies.