Adding to what Lolix said, many of the event units we've had are niche units anyways, not core team members. Only ones in recent memory that are full-time team members are Imelia, Shuka and Claudia, at least assuming you don't have their premium counterparts.

Azami: largely inferior to the constantly available Saki, ninjas are sort-of niche anyways (though ranged on melee spot is becoming increasingly useful)
Beatrice: skill is situational, but the rest of the pirates above silver are premium so she might fit in
Marie: situational gatekeeper
Momiji: samurai even with buff are still tricky to use, and her skill does let her serve as gatekeeper if you're lacking
Shao: Archer with assassination is useful but % chance means you can't rely on it so it's often better to stick to Spica and Bashira
Rita: Archer with very niche skill. Again, for general archer use, Spica and Bashira are better
Waltz: Dancers are situational and most of the units they really help are premium (Bashira is only one I can think of that's Gold+ and super-benefits)
Memento: Necromancers are situational as well, only useful if u need the cheap, disposable blockers to slow down enemies while ranged kills
Emilia: Valkyrie that is outclassed for her job by any other Valkyrie besides Elaine due to skill not really being suited to Valkyrie roles
Elva: looks like she'll be pretty damn powerful but her cost will be absurdly high without soldiers/Valkyries generating UP like mad