Most players are doing PP just for the stupid Powerplay-only modules.
Make them available without the need to participate in Powerplay, and you'll see a massive drop in the participants. (I would love to see that happen as I can't be to go work on any of the PP faction.)
Any ideas on forcing players into Open only will not work. Players will either find other methods to avoid other players via out-of-game methods, or just drop Powerplay. The rewards are too low for the amount of time you have to sink into. If you are doing prep/fort, then you will have to sink more credits than the PP will reward you with.
Players hauling for merits have a lot to lose (if they pay for more cargo to haul) versus the players that will interdict and shoot them.
They will also waste more time and effort in a hot/popular system while playing in Open for no discernable rewards vs doing it in PG/solo.
Players doing undermining in Open are also doing it for no discernable rewards vs doing it in solo/PG (except for the thrill in playing against a real human behind the controls). Fittings for anti-NPC is different, and also much more lenient versus fitting for anti-player that can still do anti-NPC efficiently. (E.g. Lasers versus ammo-based weaponry. Bi-weaves versus Prismatic with SCBs. Hull-tank vs shield-tank. Time before RTB for anti-NPC builds in comparison to anti-player builds.)
Open is its own reward for players craving the player with player interaction, and should only be played together by like-minded players. Players should not try to hamstring the gaming experience for players that prefer to play in solo/PG.
I did think of suggestions to incentivice players to participate in Open play, but it will probably be a waste of effort.
Having a bonus that matters will only cause more displeasure from the Solo/PG crowd. It will also lead to more exploits that will make the Open enviroment to be as devoid of players as Solo (block list, network configurations, bandwidth throttling, etc), which makes it pointless to limit the reward to Open only. And if the bonus is so small that it doesn't do enough, solo/PG players will stick to solo/PG.
And in the end, what makes players (most, not all) avoid Open (or just straight up PvP) is the hard grind for credits.
Seeing how FDev goes about nerfing credit mills, and the cost of ships (and their A rated modules) that are medium-sized and above, it is not hard to see why players are so risk-adverse.
Say, a A-rated Vulture will probably cost around 20-24m? That will be around 500k re-buy? Looks like a small sum of credits, but for players who do not know how to earn credits via non-combat means, it will be a grind. Don't even mention ships like the Fer De Lance, or Federal Assault Ship, or the Alliance Chieftain.
Fun to fly in, but a pain in the when you lose it.
And even if you know the way to grind credits via non-combat method, doing it in Open will risk not only your non-combat ship, but also your mission objective. Which upon failing because you died, will cause your reputation with the factions to drop. Which will also means you lose the access to the better paying missions, and have to waste more time to gain the rep back, which again if done in Open will be a risk to lose even more. Not to mention that you have to pay for the loss of cargo too if you are doing hauling missions. (Do you see where I'm going?)
Open is a novel idea, but implemented in a wrong way. Right now, it doesn't do enough to protect players who do not want to participate in non-consensual PvP activities.