So what do you think needs to be balanced and why? From my own perspective given that there are no levels, no end-game content, no ladders, no leader-boards, no Armoury, no MMO Champion/World of Raids which means there is no race to zero other than whatever an individual sets as their own targets.
How stuff could balanced is an open question and is interconnected based on other features.
For example, you could keep the solo/private/open toggle as is, but balance in another area (such as NPCs, access to different markets, etc...). Or you could remove said toggle (or just change how it works) and not bother "balancing" using the other levers I just described.
Just so I am 100% clear, as I believe some may read into this differently. None of this alludes to the removal of any game mode or the ability of a player to play in any of these modes. It may however, change how they decide what mode to choose, and why they may choose it, and when.
In terms of the why. To keep things brief, the scenario I outlined a number of times and in my very last post. In short, pve in solo/private, pvp in group, which I think would be bad for the open mode of the game (ie, turn it into an arena/bloobath/etc...).
If you think that outcome is good (which I am not suggesting you are) then I guess there is really no discussion to be had, we simply disagree on whats good for the all open mode.
Also, please remember, I am basing my points on those 3 areas I described
here, and yes I am fully aware that stuff can (and most likely will) change.
edit: not sure if this answers your question. do you want to know why I think the current mechanics will lead to the scenario I have described? I think I have done elsewhere in this thread, but happy to do so again if required.