1) An expanded and more flexible Traits system.
The concept of the Traits system, as something that gives a gameplay effect based on how you actually play the game is a neat idea, but in Original Sin, it is limited to only trait-pairs from Dual Dialogues which grant one of a pair of opposing bonuses, and it's always the same pair. It may not always be the pair appropriate for the job, maybe the traits appropriate to the current situation belong to two different trait-pairs.
Because all the traits are in pairs, it also means that you can't have stand-alone traits, and that drastically limits what you can do with them. You can't have a "Careless" trait that you get from using AoE attacks with allies in range, for example.
2) A more flexible and dynamic skill system.
Right now, skills are only considered part of one school. I'd like to see a more flexible system where skills can be "either-or" or "and". So Phoenix Dive, for instance, could require EITHER Pyrokinetic OR Man-At-Arms. The skill "Flamenado" is a Fire-Air spell and requires BOTH Pyrokinetic and Aerothurge. A piece of armour could require Strength 11 OR Constitution 11 (for those guys who only want to be a tank).
That does admittedly sound complicated to create and balance.
