A lesson you should learn from Where Winds Meet.

This is probably one of the games that took me by surprise the most this year as I had 0 expectations from it.. actually scratch that... I had negative expectations from it.

They approach one aspect in the best way possible and I think this specific thing would be incredibly useful for GGG to look at and learn from. Now what am I talking about?

Boss fights in multiplayer have different skill variations compared to solo play and are adapted to said different environment in mind so that it doesn't become trivialized.

In other words, they have correctly assessed that you can't scale difficulty with just health and damage numbers alone.

And this part is incredibly important not only for multiplayer, but for difficulty scaling in general. You should really look into scaling difficulty in this way in the game as well and not only by damage numbers and health pools.