After finishing the Myth 3 campaign, here are the main things that I think would make the biggest improvements to Myth 3 (if anyone knows how to mod the game to try these fixes, then I'll be keen to see how it works out):
1) Trow Iron Warriors. Almost all parts of the game that are bad, frustrating, or borderline unplayable, are because of this unit. Just circling them with Damas while you slowly whittle them down with ranged units is tedious and awful.
I'd try just reducing their HP to about 20% of what it is now. Yes, they should logically be tougher than normal trow. But this is just ridiculous and unfun. Other solutions are worth brainstorming, but this solution seems like the quickest and simplest to try.
2) Fix the bug in level 14 Bonds of the Oghre that prevents most of the oghre from being moved away from the temple construction site. Otherwise, point 1 above will probably go most of the way to improving this awful level.
3) Level 15 Rebellion. Hopefully point 1 above will go a long way to improving this level. Otherwise, tweaks should be tried to perhaps the timing and number of enemy spawns. I think the map layout is fine - plenty of tactically interesting terrain.
4) Fix the bugs in level 21 Heartstone of Nyx where the trow no longer flinch and stop moving for a moment after being hit by a dwarf bomb, and the bug where the trow aren't actually frozen in place from Myrdred's spell at the start of the level until they become unfrozen after a timer. They whole premise of the level is that you are meant to be working against the clock until the Trow unfreeze, but they just start off unfrozen, so that should really be fixed.
There are also definitely balancing tweaks to make to most of the levels. In particular, I think most of the "defend against waves of enemies" levels are way too frantic to play without using the time-slowing feature or playing co-op.
But I think the above four points are the main ones holding the game back and making it painfully clear that it was not playtested sufficiently.
Really, it is mostly the first point.
If someone could come up with a mod or a fan patch just for my first point, I'd be very grateful.