Some of the problems I run into while trying to make Berserker and Skirmisher better relate to Power Creep, indirectly. I am not trying to make them MOAR POWERFUL on purpose, but how else do you make spending your points from level 30 to 50 MATTER?
The current state of Berserker is such that you could beat the game with the points you have at level 30, if you had the right gear and made good decisions. It is enough to have all your key talents where they need to be. So, naturally, if I make any abilities more worth putting points in past 30, that is Power Creep by definition.
In my opinion this is inevitable. Either ToME leveling largely stops mattering at 30, or characters over 30 are going to be more powerful because they continue to gain interesting and powerful abilities while leveling all the way to 50. You can't have it both ways. Gaining 5% stun resistance for going 5/5 in Berserker Rage is one of the BETTER options you have in your later level ups, and if you think that is compelling and do fistpumps over 5% stun res, you make me sad, and I will have no more of you.
I believe the lack of fun character customization post 30 directly contributes to the slog that is the last half of the game. There are other factors, but when you take away the joy of getting tangible rewards for leveling up on top of everything else, it just pours salt in the wound. I would like to do work to make Orc Island less repetitive at some point, I hadn't realized how much love it needs until this weekend. Right now though I would like to make Berserker and Skirmisher good examples of dynamic and interesting classes with more room for player specific customization. That means more skill trees with good abilities, which necessarily means more power because there will always be benefits for having 1/5 in any skill, even if the rewards for 3/5 and 5/5 are greater.
How do we solve this? For one, enemies with classes like Berserker and Skirmisher will be more powerful automatically, leading to more interesting fights

The only solution is to up the difficulty of regular encounters, especially in Normal mode. Higher difficulties tend to spawn more monsters with classes, and gives them more classes as well, so they should adjust somewhat on their own after these power curve changes.
You may think, what is the point of making players more powerful, then monsters more powerful? What do we stand to gain from this Power Creep? Diversity of builds, meaningful player decisions even in the late game, and a more compelling and individualized ToME experience. Once the groundwork for that is done, the Power Creep will stop, and difficulty can be tuned to make up for it. It is not an endless cycle.