To get the 5th perk slot idea out of the way: you have a new perk slot Obviously but it can only be certain perks such as Hex perks/boon perks or perks like Open handed/dying light. Perks with effects pretty much, Residual Manifest/blood echo.

Any other type of perk won't work.

Now you can still use the same perk with the other slots but the 5th slot is very limited.

Stackable perks: as long as it's not one time use perks like appraisal, Hex:pentimento etc or perks limtied by it's unstackablity such as prove thyself, shadownborn, Lightborne or open handed then you can stack them.

Wanna waste all 4 perk slots with plunders instinct/hoarder? Now you can! The meme bulids would be insane.

Obviously the idea would need more balancing but I think it's a good idea.