As i wrote in another post, I like Vader as he is and only problem (maybe a little) I see in cheese characters like Wat Tambor (Thrawn too, but he is one trick pony - so no problem).
Wat come in june 2019 and after his release was relics and OP characters. With a small upgrade to strenghten more sepparatist and weaker other faction in skills like Chiewab medpac or bactoid shield generator.
Yes Rey probably appreciate in leader ability something as tenacity +1 % for every light side and + 5 % for every ressistance (including herself) would be nice or maybe some upgrade to her stacking mechanics.
Possible interaction between Vader and SLKR to infuse his ultimate faster maybe a problem but I dont saw it in practise... (a possibly solution - Vader cannot against merciless massacre crit but have higher dmg in general)
Vader zeta is worth and very fun, however if you dont have many free zetas I will wait a week :)