-Archer: Arjuna (dark skin, Hindu traits and clothes, paired here with Karna) -Lancer: Scathach (red spear kinda like Gae Bolg, bodysuit just like the hound of Culann) -Caster: No idea, but I guess it'll be a "fun" character with based koyasu to voice him. -Black-skinned Berserker: Darius III (almost three meters tall just like in Iskandar's description, yells Iskandar's name) and its design kicks major ass, like an Ãœber-Xerxes. -Blue-haired Berserker: Everyone seems to think it's Caligula, and so do I. -Rider: No idea and I don't give a fuck about that scantily-clad harlot. I swear to God Type-Moon, if that's another one of your stupid genderswaps...
Popular guesses for Caster are currently either Faust or Mad Hatter, I'm actually on more Mad Hatter camp because of the watches and scissor.
As for Rider, the vast consensus is Miyamoto no Yoshitsune due to similar armor, Yoshitsune's famous feat with how he rode his horse down a steep cliff, and having a brother.
I'm not all too bothered that she's another genderswap rather than that she's a dumb ripoff of Shiba Miyuki from Mahouka(same hair, same seiyuu, is mentioned to idolize brother even though the real Yoshitsune doesn't), which I absolutely loathes for her literally having no other character trait other than brother worship.
You're right about the Mad Hatter, that seems to the most plausible hypothesis, though it looks more like Burton's (awful imo)version. All those colors on the Mad Hatter make me want to puke, seriously. Good old first-era Burton (from Beetlejuice to Sleepy Hollow with arguably Corpse Bride) is now lost in limbo...
About Rider, yeah, now that you mention it, it definitely seems like that. And that's a shame, from a certain point of view. For Pete's sake, I like Saori Hayami's voice, but I swear the only character I heard her on that didn't make me cringe was Sawa from Taritari.