The main thing I can think of, and this isn't quite power customization, is a way to deactivate visibility or turn down the business of some power effects. Some people cannot team up with Force Field heroes because all the field wavy effects give them a headache. Similarly with stacked auras, glowy abilities, etc. It's difficult to say what exactly to do about this? Some change in the graphics settings to turn down the brightness or simplify the patterns of certain things? I'm not sure, but it should be thought about.
Apart from that, force field effects in general tend to be invasive. It is important to make the area of the bubble visible for AoE abilities, but the wavy effect tends to be distracting or even disorienting in large enough amounts. With that in mind, I'd like to see two things: bubbles that are smooth or have a more subtle, static pattern (hexes, maybe) and an FX for individual effects that is not the bubble.
Other than that, while lower priority I'd like to see some additional flexibility for pets. Master Mind customization, of course, but also for things like Control pets, the defense drone from traps, etc. This is obviously a larger undertaking, but broadening the models available would be neat.