I don’t know how useful that post will be, but here is my 2 cents on stuffs I felt missing for end-game componentization (is that even a word ? Now it is :D), mixed in with some observations on the current existing stuffs.
Apologies for formatting, verbosity and likely typos. I’m pretty tired at the time of writing.
Weapons/Off-Hands/Shields
They have their own ecosystem and there is plenty of choice. There is always room for more obviously, but no specific complains or suggestions here.
Jewelry/Accessories
They also have their own ecosystem as well and are mostly alright.
I have one minor caveat with rings: If your build’s energy and CC res are fine already, and your bleeding and vit res are fine too… you don’t have a goto offensive endgame option like seal of annihilation/arcane spark.
Soul shard/Roiling Blood/Vengeful Wraith/Mark of Illusions are niche when it comes to pumping damage.
If such component could include some reflect resist as well, that’d be wonderful, but maybe I’m asking too much 
Armors in general
As far as endgame go, there is 4 components that can be applied to any which are really relevant:
- Bladed Plating
- Enchanted Earth
- Spellscorched Plating
- Ugdenbog Leather
The other ones that can be applied to anything are mostly superseeded by these, even if one could always find counter-examples.
What I’m interested into is the rest, i.e. slot-specific components, because that’s what makes the puzzle solve to fun for me. I know I can always default to enchanted earth on anything if everything else I need is already capped. But I like to have options ^^
Head and Chest
Bunch of unique interesting stuff for these two slots, with some overlap with each other and other pieces, it’s absolutely fine for me.
Gloves
They have their own unique set of offensive components, and I like them, so fine here too.
Boots
They have marks of the traveler/mogdrogen’s which are unique. Now that we’re a bit less shy on max movespeed, maybe a new component for that would be great ? 
Shoulders & Pants
They’re the default target for armor absorption.
And… I guess it’s ok since we need to put these somewhere anyway. At least for shoulders, OA or Health is always great. But for pants which don’t have access to living armor or sacred plating, there is an issue: If the rest of the build already cover all/most of your armor absorb, you’re stuck with the default choices.
Something more unique than just an armor buff would be nice.
One thing that fit thematically would be slow and entrapment resist (or some other CC res, but ring components exists), alongside a 12% absorb.
An other thing that could be nice (not too sure about usefulness though) would be an option to reduce DoT duration (mostly poison and bleeding).
Belts
They’re the first thing you put a component on when leveling. But in endgame, it feels super generic. I typically just slap ugdenbog leather or spellscorched plating for whichever help resists the most, then I start thinking seriously about the other components.
Now, one thing that would be VERY fun for belt components would be some conversion component. That’s fresh, that’s unique as far as non-weapons components go, and fit nicely with belt being a typical conversion slot.
Something like a 15% conversion, matching the existing ones on farmable +1 belts. The main idea being that with a belt converting 40% to 60%, and most other conversions on gear being around 30%, you can choose to clean up you conversion on your main damage type if needed, while sacrificing the usual defensive bonuses you’d get form that slot.
Anyway, these are the few ideas I had written after setting up and playing 20-ish (maybe 30 now?) endgame builds.
Thanks for coming to my ted talk.
tl;dr
I feel some components slots would benefit from more option. Suggestions are:
- [Ring] A ring component with versatile offensive option (all dmg/crit dmg/OA/speed/…cdr?
), and I would personally love a reflect dmg reduction option on it as well. - [Pants] A +12% or +8% armor absorb pant component with slow resist and an other defensive property.
- [Pants] A reduced poison and bleeding duration pant component and one or two other defensive properties.
- [Belts] A collection of belt component with a 15% damage conversion that match existing conversions on farmable green belts, with each an extra property.