I think all of these ideas should be available, with the ability to combine them all. Delegated, replaceable PoW on pubkeys or event ids.

This unlocks some interesting functionalities, like being able to set a higher bar for entering a community / flagging content, the difficulty being adjusted by moderators based on the amount of resources they have. If the community sees good quality content with low PoW, they can hive-boost the PoW to raise the content to the front of moderators' eyes. Good content could stand out this way.

Higher PoW is also a nice way to kickstart an anonymous profile with no web of trust at all.