The Game Designers should take a look at Raid: Shadow Legends
I say that not because it is a particularly complex game, but because of some of the ideas that keep the game addictive despite the simple gameplay.
1. Having the 3 schools of damage, and one neutral to everything, make balance irrelevant. There is a hard counter to everything. Even the strongest character, is individually almost helpless against its hard counter, in terms of damage out and in.
2. There are constant arbitrary competitions with players put into brackets automatically by power, level etc., all giving rewards, fairly deep into the field in each bracket.
3. There are a lot of quests, missions, challenges and so on to do. These get you deep into the endgame.
4. There are constant free gifts, particularly once the game is established, that makes the content easier, and faster, but doesn't allow you to bypass it entirely.
So I mention this, not because the game itself is particularly deep or interesting, but because these mechanics enhance what is a very simple game. The way they can keep this game so balanced and rewarding for so long, for new and old players, is really remarkable.
However point 1 is key here. No single character can dominate. I cant help feeling that the introduction of a third category of damage, say "Heart", was introduced, a proportion of which caused aggro, and then making the classes either magic, physical, or heart (tanking), would take the issue of "balance" out of the discussion forever. Heart could be strong against physical, magic strong against heart, and physical strong against magic.
From a games design perspective you can do so much more with a triangle than a line.
