@Cahir
For Diablo 4, a legendary item just means "rare, with a legendary power", so yeah, they don't feel very unique. However, in World Tiers III and IV onward you can find Unique items. These have their attributes and powers set in stone and cannot be modified. So, when you drop Harlequin Crest (yes, the Shako exists in D4, saw the screenshots), it will always have what you expect it to have.
As for managing legendary powers, aspects etc, ask yourself if you are going to roll another Druid anytime soon. Because if not, you can easily scrap the legendary items that you are unlikely to use in the near future for materials. I would not necessarily recommend extracting every aspect you can find, because in the endgame (World Tiers III and IV) when you get access to higher tiers of items, you cannot use lower-tier aspects on the higher-tier item. Namely, you cannot use aspects from "normal" legendary on Sacred or Ancestral legendaries.
Another bit of info I have for you, legendary items can last quite a while since their legendary power can be just that useful. From all equipment slots, the weapon starts falling behind the quickest, so for weapons, I suggest imbuing re-usable powers from Codex of Powers on new cool rare weapons you find that work well with your build. Also, for my current knowledge, you cannot just swap legendary powers between two legendaries.
As for enchanting your gear and changing affixes, yes, you can do that. However, the costs of doing so are increasing very quickly with each enchantent attempt, so if you keep doing that you'll go banckrupt
As for opinion pieces, I do agree with runewords. As for your first idea, I don't think that would work out because it would defeat purpose of finding better gear. As for set items, they are rather problematic. In D2 only few sets could be worth making builds around in engame. In D3, sets utterly derailed the game and are responsible for insane power creep, something I do not wish to see in D4 ever. Needless to say, implementing sets well is going to be tricky. It would be cool if they could be implemented without feeling underwhelming or dominating the game, but I don't believe it happening if earlier Diablo games are of any indications.