I think the only characters I could see not being replaced are Siegfried, Cervantes, and, iconic bull aside, Mitsurugi.
Nightmare doesn't count as being "replaced" or not, because many people can become "Nightmare".
Astaroth, especially, could have been EASILY replaced, but what saved him was Maxi's appearance, and with Maxi not being able to age, I'm sure Astaroth will NEVER be replaced in future installments. Ivy will never be replaced either since not only is she scantily clad. She's highly intelligent and she's asexual (I assume), so that means no kids for her.
Everyone else......
Taki=Pretty obvious she was going to be replaced. However, Natsu just needs moves that she can call her own, besides teleporting. She has her own personality though. Leixia is the same situation as Natsu.