I thought of Armored Core (surprise, right), so then I get Dark Souls which is the obvious choice and Enchanted Arms, which I didn't even know FROM made. Those are the three I've played from them and the only other one I can think of is King's Field, which I haven't played.
I would expect Nineball, White Glint and maybe some fan favourites like Berlioz/Supplice, May Greenfield/Merrygate and Jack-O/Foxeye from Armored Core, Karin, Raigar and Yuki from Enchanted Arms (heavy melee, light melee/water elemental, light ranged is a decent spread of styles), and Dark Souls is a tough one, but I'd like to see Gwyndolin (ranged caster), Solaire (all-rounder), Smough or Tarkus (tank melee), and Oswald of Carim (would have some great mechanics). If Artorias wasn't such a badass I'd include him too, with Sif as an assist. As far as other assists, Golem for Tarkus, Siegmeyer of Catarina for Solaire (or, Sieglinde for Siegmeyer would be cool too), Ornstein for Smough and Velka's crow for Oswald.
Just realised how fucking awesome this game would be.