We're pretty much even I think right now between the good, the bad, and the neutral so you can choose whichever side you desire. As for niches hmm well again we might be even on those so really it's up to you. Gravity would be interesting but my problem would be more on how he/she recharges?
If you need help I know the Infamous wiki names a few powers that have been mention that you can look at:
http://infamous.wikia.com/wiki/Conduits
Gravity is on that list so it is possible. As long as you don't make it overly powerful I'm fine with it. Only question I would have would be how they recharge it since all conduits seem to need the ability to absorb what there power is. So would it be like he takes away gravity a few feet around him for a few seconds when he charges before it's reinstated back to normal when he is charged?