To echo a lot of people, PvP would need to be less about fighting each other and more about completing wild objectives. Especially if they were designed in such a way that the side that gets the most kills isn't necessarily the side that wins. The objectives in the PvP zones on a much smaller, instanced scale is kinda broadly what I'm thinking. SWTOR's PvP when it wasn't just a 4v4 arena match was really fun to me and I feel like those sorts of game modes aren't out of the question in CoH, assuming development time was no object.
A big thing for me would be if the difference between a reasonably thought-out SO build and a decked out IO set build were minuscule. One player being numerically advantaged than another is a big switch off for me. It should be close enough that someone trying PvP for the first time can easily KO a hardened veteran with a bit of luck. I'd like to be able to jump in and feel like I'm making a contribution without having to know what I'm doing.
On the back of that, I'd want killing to be easy and dying to be really forgiving and neither have a significant impact on how the match comes out, so that the action just keeps going. I'm thinking like the pace of Unreal Tournament games.