Strahd is who I'm having the toughest time with. I have him pinned down as a shiny Rapidash or Mewtwo (with sergei being a regular Rapidash or Mew).
Rahadin is a sceptile that can mega evolve. That came to me in an instant.
Ireena I know for a fact should be a Kirlia because her arc has usually revolved around her role as a Tatyana reincarnation and her current life as the daughter of the Burgomaster. The opportunity to evolve into Gardevoir or Gallade is great. Plus the wedding motif they both have is perfect.
Ismark would be a Pikachu, mainly because he has become the party mascot in most of my previous campaigns. Plus he's blond.
I won't go crazy indepth about the rest of my choices so far, but I'll list them:
Madam Eva: Xatu
Ezmeralda: Blaziken
Van Richten: Absol that can disguise itself as a Sylveon
Ludmilla: Mismagius
Anastrasya: Ninetails
Escher: Smeargle
Volenta: Marowak or Scyther
Bildrath: Kecleon
Parriwimple: Machoke
Morgantha: Drifblim
Offalia: Dusclops
Bella: Gorgeist
Baron Vallakovich: Slowbro
Victor: Slowpoke with a kings rock
Fiona: Alolan Persian
Stella: Meowth
Blinsky: Mr.Mime
Abbot: Dragonite with a Dragoninite
Vasilika: Magearna
Clovin: Weezing
Mad Mage: Delphox that would normally have a Delphoxite, but bro lost it
Pidlwick II: Porygon Z
Arigal: Scrafty
Arabelle: Happiny
Baba Lysaga: Hypno
I'm lost on everyone else, pretty much.