Powerplay consists of low quality gameplay for low rewards via a UI and systems that hate players. Open at least would make each haul run actually worth paying attention to, and that groups would have to add more tactical roles.See, I'm not sure about that because quite a few of them seem to specialise it more towards the tastes of the few already doing it. This being one of them. A suggestion that mode-restricts the activity to open is going to have the opposite effect, it's going to remove the players who want to do powerplay and feel like they're supporting a faction in the metagame but who don't want to play with other people or can't play in open.
If you want suggestions to bring more people into Powerplay, you want to look at reasons people aren't doing it right now and focus on changing those.
I very much suspect "too much hauling" is a massive one. It's certainly one of the big reasons I'm not bothered with it. Like people will do hauling occasionally for a community goal but there they get paid in personal progress for the hauling and have a clear understanding of their effects within the goal. But if there was always a hauling community goal on then people would drop out of doing them after a couple of weeks.
Powerplay for most factions is a hauling community goal that never ends and you have to either wait or pay to skip the wait do it, getting no personal benefit for your trouble only vague merits buried in a menu somewhere that lead to a "wage" which is literal peanuts.
One of the biggest things it needs, if it's not going to be completely redone as an exclusive PvP mode, is a wider range of activities to generate PP progress, so that every major gameplay interaction (with the possible exception of exploration which is inherently in unclaimed space) has some effect as long as you do them in the right place for your faction.