Because of the way Rex, Cyber-Hound's second and third abilities are worded, he can continue to use abilities of exiled cards even after leaving play
Aphemia is solid here since the Zombie tokens matter in combat against the dorky Mono-Black Aggro creatures, and black creatures are kinda nice if your opponent shows up with Self-Inflicted Wound
Jeska, Thrice Reborn will let you triple Bruse Tarl's combat damage each turn
Kenrith, the Returned King A little bit of everything, all of the time Are you someone who likes abilities on creatures you can activate at any time, and also someone who likes five-colour decks
Innistrad: Midnight Hunt was chock-full of Cube goodies, and Innistrad: Crimson Vow looks like a fitting follow-up
When you are reliably finding one and winning with it when you just have three then you don't overly need a 4th