Off the top of my head, a valid Powerplay engagement would be attacking an opposing power whilst in your own or their own control or exploited system.
Potentially it could expand to any power on power engagement, it's something we'll be looking at as part of karma.
Thank you, Sandro.
If you decide to go for anything less than a complete pledger v pledger exemption (assuming not of same superpower) then I would ask you to consider broadening the terms of your first paragraph.
Specifically, if you go with Controlled and Exploited systems as the legit battlegrounds, I'd ask that any pledger v pledger violence within those systems be exempt.
Example: Hudson Cmdr and Archon Delaine Cmdr are both attacking ALD Cmdrs in an ALD system. Whether they choose to ally, ignore each other or attack each other should not result in negative karma. Similarly, a Patreus Cmdr should be able to repel either of these non-Imperial invaders even though it is an ALD, not Patreus system, without penalty.
What I mean is, there's no need to stifle the dynamic nature of Powerplay PvP at its most interesting, exciting and unpredictable, via negative karma.
However, I'd like to reiterate that offering a galaxy-wide Powerplay exemption really could both give PP its shot in the arm and offer the game its unofficial 'PvP-on' flag.
The more people pledge, the less unwanted heat for those who don't pledge, if you see what I mean. If a CG is in a non-PP system, the ideal scenario for a non-PvP guy who still plays in Open is that the gankers ignore him because they are pledged and they have pledged targets (both karma stick, and pledged carrot, as it were).