What I have learned from playing Guild Wars is that skill synergy can take a game to a whole new level of fun and interest. It still keeps me hugely interested in the game for after two and a half years now. And then I'm only talking about the "skills"-topic and not yet about the "story" etc.
What I mean with skill synergy is that different skills in the game work together and that you can make different combinations that can become very efficient. For example a few good melee-weapon combo's. Or necromancy skills that can raise skeletons/any other minions supported by skills that can heal them, support them and enhanche their damage.
Or team synergy if you are able to take companions with you in any sort of way, and you are able to chose their skills in any sort of way. You can make everyone a "jack of all trades-master of nothing", or you can find out your own combination of skills, weapons and companions that owns the **** out of your enemies. Also, that could imply that if you found a great combination of skills for a certain area, it sucks for another area. And that's good, because that means you will have to change the way you play from time to time.
I know this has very much similarties with Guild Wars and in DD for example you had a large list of skills you could choose from anytime.
But I find it a nice idea to implement. Because besides from the story (which I trust you on, Larian, make it great
), the skills and everything you can do with them will also keep you interested in the game for a long time.
P.S.: Let me give you a very simple example that could be placed in DD. You use Freeze on a monster so it becomes frozen. Then you use my modified version of lightning: it does damage, but it also gives extra damage if there's water involved (electricity+water). So Freeze+Lightning could be a simple but fine combination of two skills. Skills could also counteract because of this. For example, if you use Freeze and then a spell that has fire involved, the Freeze wears off because of the ice+warmth=melting thing