Now dont get me wrong, I am not suggestion that when you start the game, you choose if you want to be "Warrior", "Rogue" or "Magician".
All I am suggesting is that at certain lvl, make it 30 or 40, you can choose new kind of skill, which isnt actually a skill, but more like a skill set.
ie. Berserker skill set, which gives you +X% damage to all hits, -X% to BC and unable to flee from combat.
This must not be anything too good, more like it could adjust how you play, not to overpower you.
And into addition to even getting this skill set, it has requirements, like: AP200+, lvl 30(or 40), etc.
The point of these is that they do not give only positive effects, but also negative which will effect your playing style.
There would be other skill sets for differend playing styles, like
Defender +hpmax +bc% -ac% -dam%
Stealth (ie. +50% crit chance for first hit (surprise), but after 5 rounds, -5dam (stacks up every 5 rounds))
and so on...
Feel free to comment and adjust the idea
Just edited out a typo