I think some characters should either have died, stayed as NPC/guests or not existed at all.
Elliot, Laura and Millium, for instance, shouldn't have been part of class VII, or, in case of Millium, I'd even prefer if she didn't exist at all.
Elliot's arc is something focused on music and honestly very forgettable and not plot-relevant at all, and also completely pointless, as he still plays music in his free time and manages to become a musician in CS3. True, Olaf is a badass, and hilarious too, but we don't need an additional playable character just to care more for him. Elliot should have been a guest in Heimdallr and escort you for the arc, like Sara does in Garrelia and Angelica does in Roer. Laura is also as unremarkable as Elliot, if not more. She could have been an occasional guest from another class (and maybe a guest in Legram), and Victor's death would have given her a few more bits of characterization.
Millium, on the other hand, I'd have preferred if she didn't exist. True, Millium has a connection with the very plot-centric Altina, but Altina gets a hella lot of development and character growth while Millium does not, and Altina could have actually fulfilled her role by herself and ultimately sacrificed herself to become a sword, without any cheap way to have her back. Then, if Millium is an Ironblood and indefinitely replicable, why did Claire and Lechter kidnap Altina instead of using Millium and then cloning her? That resurrection is a cop-out, and that comes from a series that nailed resurrections perfectly with Crow and Osborne. If it were for me, I'd have Altina sacrifice herself (for real) at the end of CS3.
Having someone resurrect after having been used for the sword would have worked if the resurrection (backup of her personality) was handled similarly to how ASOIAF does resurrections: after resurrecting, her personality is no longer the same as the one she had before, she lost part of her memories, and is obsessively focused on what she was doing before dying. Altina or Millium's personality being backed up in another body would work, but it needs to be corrupted and overall not exactly the same as before. If Millium never existed and Altina was used this way, her arc would only be furthered by the conflicting state of having been a robot, and her newly-found emotions being corrupted: Rean would have to try even harder to make her finally gain humanity, possibly furthering her arc until Hajimari and making her the de-facto protagonist of the "Western Erebonia arc".
Resurrection must have a price to pay. Crow in his first resurrection had one: that he would die again once Ordine is absorbed or the Great Twilight ends. Or it would have if they didn't find a bullshit excuse to circumvent it.
To people saying Gaius shouldn't exist, I think Gaius needs to stay, and be in Class VII, as he brings a new perspective and allows us to see a foreign culture with a huge relevance in the worldbuilding (Dreichels started seeking warriors in Nord). Not to mention his Stigma. Not to mention that I liked his personality and his devotion to nature and wind. He's basically Elliot and Laura done right, and if they didn't exist (or were NPCs) he would just get better.
I also think Sara should have died in CS1, against C (either in Roer or in Trista at the end). She's a mentor figure, and her death would leave Class VII on their own, forced to experience the harsh world around them without any mentor figure to protect them. This would be the foundation of a character arc in Rean, who should have realized he can't save everyone, someone has to die and there isn't any way to avoid it. Being killed by Crow (her supposed student) would make C even more threatening and Rean's rivalry against him being in order to avenge Sara would make Rean feel conflicted whether or not he should forgive him.
In CS2, Scarlet should have died. As I said, Rean should have learnt he can't save everybody, and as such not go out of his way to save Scarlet. After all, she doesn't even appear in future games, so her living is absolutely pointless.
In CS3, Angelica should have died for real. Likewise should Toval and Victor (not Olivert as he's the series' Cid, so at least once in every arc he must appear).
In CS4, I would have had the party actually kill Lechter, Claire and maybe even Sharon and George. Also Crow. The first resurrection was really well-written, but Crow technically lives on borrowed time now, Rean knows it, and it would have been better if there wasn't any way to prevent his second death. George, as a reluctant villain as he has always been, would want to kill himself for what he has done.
I did love the CS games, but I think it's strange that everyone is so easily forgiven.
Deaths: Sara, Scarlet, Toval, Victor, Angelica, Altina (resurrects, but corrupted), Crow (dies for real his second death), Lechter, Claire, Sharon (possibly), George (possibly)
Guests/NPC: Elliot, Laura
Not existed: Millium
What would you think?