Node-based UI for effects.
A node based environment, should be implemented esp for basic tasks like applying the same effect to multiple layers, alpha mattes, value parenting (like the current pick whip in the expression options), Anything that you can currently do using an adjustment layer (but would be selective), I'm sure many more uses. You could use it in tandem with the layer system, but would be an easy way to do these kind of tasks without copying and editing multiple instances of the same thing.
Gareth Blayney commented
If I want to apply a glow to four layers using an adjustment later, I have to pre-comp. If I then want to adjust the glow, I have to go into the pre-comp, make changes then return to the main comp to see how they work. And over and over - if we had a node system where I could simply pipe that effect to the four layers and tweak, it would just be so much better. The lack of a node system is making AE feel old.
It would be nice... but Adobe are happy to not innovate. If they had to be innovative there's no way they could rush out a buggy version every year! Quality company