I'd like to see the powers become different as they get more powerful. In Diablo 2, the only difference between a level 1 Lightning and a level 20 Lightning was the damage. What I'd like to see happen is when you get a power to a high enough strength, it would begin to inherit some of the properties of other, similar spells, proportional to how many points you had put into them. For example, if you had maxed out both Chain Lightning and nova, then you could start to get occasional "Sparks" jumping off of the nova ring and shocking enemies that would normally be out of reach.
Also, as an extension to the last idea, if you put points into the "Mastery" spells (assuming that they carry over to the wizard class) you could begin to merge skills at will, creating your own "Hybrid skills". To give this power a drawback (the game needs balance, too) each "hybrid" that you create will incur a mana penalty, where you lose a small (or large, depending on the hybrid's strength) amount of mana simply to have the skill available.
For the other classes, I'd like to see the witch doctor able to make alchemical potions, preferably offensive, but maybe healing and/or defensive, too. The offensive potions could be thrown, like in the previous game, but could also be used in different ways. They would not only have short-term effects, but rather have long-lasting, more destructive effects. Explosive potions would not only detonate, but spread burning tar to ignite the area around it, and any enemies who happen to walk over it(They don't just take fire damage when in the fire, they are set on fire). As an alternate way of using this, you could open the explosive potion and use it against a single enemy, drenching them in burning tar and making them leave a trail of burning liquid. I have no good ideas for the barbarian, unfortunately.