Hot take: uniques should be unique.

What I will say applies to every Hack n' Slash I know, but I would be thrilled to see one, e.g. POE2, implement a solution.

One thing that makes unique items not feel unique is that it is often much more easy to acquire a unique than a good rare (even in SSF), and that you end up with multiple copy of many unique items. This feels wrong. Imagine that unique become unique: once they have dropped in you account for this league, they will not drop again (even for other characters in the same league); and you can never have more than one copy in your account, even by trading (except possibly in permanent leagues; or you might have to choose between your copies when getting the unique out of the temporary tabs).

Of course, keeping some crafting of uniques is important, so it would be important to have somewhat affordable mechanism to craft and recraft, and reroll with slightly more control than divine orb, at a steep cost.

It would also open some possibilities to design unique rings and 1H weapons, that would be too extreme in two copies, and it could enable more build-making uniques without breaking balance. Dropping a unique would feel that much more special: even if it is not one you want, its value for trade would be much higher.

If sets were to appear, it should also apply to them to make them special. I would then tweek the drop chances in favor of items not yet dropped in sets that started dropping, at least in SSF.