to every action must be given the option of a re-action. Powers with fewer players must stand a chance against Powers with large numbers of player, if they play smart.
to make things more volatile, you just need to expand the actions a Power can do and add an objective for all Powers instead of top 10 and the bonus of top 3. Like a championship. I would much more prefer total war, but I guess it would be at this point too hard to implement.
fortifying by killing NPCs is the same action done differently, so it would hurt Power Play. An example of expanding your PowerPlay Horizon: doing missions for a specific faction of that system would contribute to the fortification trigger. You have changed the number of merits done with undermining "to have more dynamism". Fortification by killing NPCs seems to be a step back. I would welcome some form of recompense for killing enemy Commanders, no matter where they are. The best reward from my point of view would be to gain some points towards unlocking the Power's module. So if you destroy a Winters' Cmdr you'd gain 1 out of 100 points towards unlocking the pulse disruptor. These would solve a lot of problems. You should write some kind of inbox warning, upon joining Power Play, that we live in a dangerous galaxy. Being pledged adds to the danger even more.
Target assault : I guess it would be some kind of Death Star, but you really need to also have a Life Shield.
here you have some ideas:
create "fortification hubs" - a hub fortifies systems in a certain radius. You create one by delivering different products. You would have traditional fortification, but also tactical fortification, by delivering different products, they could even be Power specific.
The same way you could create an "undermining strike base"
This would create tensions and would be a reason for gaining certain system, which will also have strategical importance and not only CC value.
I would also introduce biological warfare and other valid guerrilla tactics. A biological strike would decrease the CC production of a certain system for one cycle. That means of course that "disease centers" would be available as a counter tactic.
Any strategy game must have some form of reference to chess. Fortification/Undermining are actions of pawns. Preparation/Expansion of Knights, the Hub would be the Bishop, the Biological strike the rook. Maybe one day we get the Queen and the King.