Should be able to make "unique" items better

Unique items aren't unique at all according to y'all's definition...everyone can get them, there's only a few hand fulls of them and you can't upgrade them besides quality and sockets. The socketing system sucks also...POE 1 you could resocket and have the runes work together...nope not in this game. Y'all ruined that for sure.

Unique items in POE 2 are just trash items, except a few ones.
When you find an unique item in act 1 or 2 and suit your class, why are they weaker than any blue item you find ?

Unique items should be far more powerful but for obvious reasons ( cf Daiblo IV) we should be limited to carry one or 2 maximum at the same time.

They are unique because most of them do a unique thing you cannot find on rare gear. It was the same in PoE1. They are good for leveling and I disagree they are weaker than blues. A few of them are what we call "build-enablers" which mean you base a character around that item. But in most cases, good rare items will always be better.

There was a mechanic for a period of time in PoE1 called Prophecies that would allow you to upgrade interesting low level uniques into high level versions. I always liked it, it would be neat to have the mechanic return in some way in PoE2.

For example, the two-handed minion bonus sword was usually level 25ish irrc. But the prophecy would let you upgrade it to a level 65 version to try and blend melee and undead necromancer gameplay together.

Something like Precursor Forges or Armories you could find that let you level up a unique might be fun for interesting builds, but that sounds like a league mechanic, not an EA mechanic.