X-perl's "Tooltip Buff Helper":
https://imgur.com/a/oH9Ji
Minimalistic feature that on mouseover shows who is without buff (pressing ALT: with buff).
All targetbuffs:
https://imgur.com/a/3QNn3
(Example: CT_UnitFrames)
Essentially it would be nice to have an addon that shows all target buffs (doesn't have to be anchored to targetframe), so you can use it on whatever UI addon you prefer.
Target castbar:
https://imgur.com/a/P1UOp
(Example: modui)
https://imgur.com/a/CqlJV
The most "standalone" target castbar addon I found so far is SUCC-ecb, however it is missing alot of spells and isn't really that customizable https://github.com/checkem/SUCC-ecb
With all the different UI addons there are certain elements that are unique or more advanced than from other UI's, so it would be very helpful to not have to install the full UI for said element. If there already are standalone addons for those three (priority on target buff/castbar) please let me know.