Funny enough, Saber Alter could have easily qualified for Avenger if it wasn’t for the fact she was implanted during the time when they still have some sort of bullshit rule established.
Alter Ego would have been just the Mysterious Heroine, or, technically speaking, all of them are Alter Ego.
What we’re truly missing is a Mooncancer Saber. That’s right, we need the unholy abomination of Takeuchi drawing Sakura Matou as Saber. A “blonde” Sakura, if we may.
Or even better maybe if it’s drawn by Wada. Given a different shade of color and a little Extra. Yes, Mooncancer Saber. How absurd it would be.