I mean realistically a lot of this shit is going away by next year. While they're keeping stuff for 4.0 I expect massive reworks in a lot of these systems and any of the weaker systems will be removed or combined with others somehow. I expect a bunch of the content will be relegated to optional content you can do if you want, with keys you can use to spawn the old shit(abyss/breach/blight/metamorph for example really don't need to constantly be spawning everywhere and the bosses can simply be available via keying).
They do need to clean up stuff some I think, there's just too many random encounters and a lot feel same-ish, just click a thing or walk on a thing and monsters spawn and you kill them for terrible rewards and some sort of unique currency shit that you might never end up using(I pickup breach shards every league but I did the bosses only once for the achievement for example, I just sell them or don't bother for the cheaper ones). It's just not very interesting. And there's way too many different types.
Syndicate at least, while confusing as fuck, is fairly engaging, you get a variety of encounters and then you interact with your board, move stuff around, make them line up in a certain way, spawn bosses you yourself put there and so on. It's far from perfect, rewards are atrociously unbalanced so I'm sure 90% of the endgame player choose the same shit and it's terribly explained or put together, but I think overall it's an interesting bit of content with some lore aspects and stuff like that.
Delve is similar, it's its own side content stuff with a lot of things, so I think it's interesting to keep.
Temple on the other hand is meh although it's getting changed so we'll see but it needs a boost to make it a major mechanic. Beasts while nice in terms of playing really don't add much and are basically the equivalent of a minor league like metamorph at this point, so they should probably just be downgraded.
I think Synthesis if it hadn't been complete dogshit would have been a lot better as a major league mechanic, although it'd need a shit ton of reworks, but there's a lot of lost lore and content from that.