Athreos is good if you want to make your enemies life miserable but its a bit slow.
Obzedat is a good beat stick too
Or you can go for Karlov and add some lifegain ETB's into the mix and kill players in one fell swoop with a gigantic 20/20 ghost that nukes key threats.
probably the 3-mana teysa, if for nothing else then additional token generation for smokestack or whatever
my favorite recursion/etb shenanigan cards in white are
Gift of Immortality
Yosei, the Morning Star
Ashen Rider (don't bother unless you can cheat him out)
Angel of Serenity
maybe World Queller for staxing
you're also in the colors to do the upcoming Bitterheart Witch + Overwhelming Splendor combo
Tidehollow Sculler, Fiend Hunter, & Leonin Relic-Warder all have their abilities printed in the old O-ring format and are abuse-able. Containment Priest fucks over cheating creatures into play and is an alternate way to abuse o-ring/banishing light's "return" ability. Grand Abolisher is probably the king of nonlegend hatebears desu
this can help recycle your creature ETBs, Sin Collector is another card worth looking at
Gains suspend X where X is the number of times its been cast before? Too easy to abuse?
suspend can be taken advantage of. obliterate a turn before my commander comes in, ez modu.
the best idea would be to have a turn cooldown. it dies, you cant cast it until after your next turn. increase the turns by one turn from here on out per time.
Those all promote certain archetypes. Discard is a boon to reanimator and madness, lifegain decks like kokusho would be happy to pay life. Labman wants to exile his library.
Mana cost from the command zone is the hardest thing in magic to cheat with, and is the best solution.
Red Sakiko?
Neheb is much, much better.
Green is much, much better.