Missing units from my wish list except GR units are below.
Eden and Memento are absolutely required for later missions, and cannot play any more without implementing the alternative units.
(If you have some black units, no problem. And no chance for me)
Chloe,Noel,Franc,Nagi and Liana are highly required. (Liana is in 'premium summon', but...)

Doubt we would get Memento or Eden with the way 'lolis' are going. Chloe and Noel are in that box as well. Franc... well I have Elizabeth so... as for Nagi I don't think she is needed, but she is a black unit so I'll probably get a perfect one from her event regardless. Liana... I doubt we will get her event because she is a premium unit for us so... sigh...