If a fortifier can't be attacked by either a capable NPC or a player, then Powerplay can't really be termed 'meaningful' at all because strategic roles can't be stopped.Not really/at all; even in the best of cases as proposed, it would still be difficult or impossible to locate an undermining player, and harder still to kill them, as they'll be fully kitted out for combat. If even haulers can high wake away with ease, as is often claimed, then it should be functionally impossible to kill an undermining player.
The biggest impact wouldn't be on systems under attack, but on systems being fortified, as haulers are much easier prey than even PVE combat ships.
But either way, you're forcing players to pvp. Or, at least, trying to do so. In practice, nobody would ever engage in it, because they don't want to, and there are so many ways to avoid it.
Being able to mess up a powers fortification makes for more strategic play, rather than having maths do it for you because there is much more to think about and overcome.
But Powerplay, especially an Open based one in whole or part this would not be the case. The largest power has about 100 systems, with the smallest 30 odd- all have a capital, and explicit activity zones. That cuts down the 99.999% down considerably- add to that the potential for a smaller BGS footprint, uncapped UM and unified fort direction a lot of players will be tripping over each other.Sure they are. They're exactly equal. All the mechanics are completely alike.
And lets be honest, the difference having other players makes is virtually insignificant. You can go to 99.99999% of systems in the galaxy, and 99.9% of systems in the bubble, and face no hostile player interaction whatsoever.