Cosmetics are fine to me, but they're a one time purchase. Not a fan of skilling item bags/flat skilling items, these will mostly cause people to boost. Imbued rings are fine with me, as long as they're sinked properly. I would prefer them to be in a minigame, or we could have both. I do have some ideas for rewards.
Teleports to wilderness locations - Edge teles are a perfect fit, the teleports that go to the wildy in the ancient/lunar spellbook could be added too.
Looting bag & rune pouch - Osrs balanced these items really well, they're perfect for the pkp store.
Skillers may try out pking once they have a friendly environment to do that in. Bad pkers or unexperienced pkers don't exactly get the nicest treatment, hell, people don't get the nicest treatment in wilderness in general.
As far as ring upgrading goes, everything that goes into the imbued ring should have a possible sink. It might be interesting to have the ring turn into something else depending on how you die with it. If you die with it in PvM, the ring is lost, if you die with it in PvP, the scroll is lost. I'd also somehow like to involve ring of stone, maybe require one to purchase a scroll or something.
Here's an interesting idea: Bounty. Let's say the currency for the pkp shop would be blood money because we're not already copying rs. You would be able to place a bounty on someone. If that someone gets killed, the killer recieves that bounty or a part of it. If you placed a bounty on someone and he gets killed you get a message saying who killed him, if you're offline at the time it will appear upon login. A board at edgeville should display all current bounties and total bounties (total amount of bounty that has been placed on you).
It may reduce the amount of toxicity in the wilderness as well. Angry at someone? Place a bounty on them. Someone killed you? Place a bounty on them. Someone backstabs you in multi? Place a bounty on them. You'll get the satisfaction knowing that someone kill them for you.
Cash bounties may also be a thing, but I'd prefer them to be pk store currency only.
You can't really prevent boosting but nobody cares about people boosting castle wars either so it doesn't really matter.
Making boosting a bannable offense would also be possible.
Boosting castle wars is of very little significance since there is already a very large amount of every reward ingame and they are by no means worth the time it takes to get them. Boosting defeats the purpose of having the pkp shop, which is getting more people to pk. That holds a lot of significance when we're talking about adding something new. If, instead of getting new people to pk more, it just turns out to be a method of giving the current pkers free shit then I don't think it's worth the time.
Making it a bannable offense doesn't solve any problems and will only cause frustration.
