Art, maybe realistic art. For the Black Knight I was going to use something like this:
Preferably we could use two pictures for our characters. Both using art. One of their faces/bodies/general appearance. And a second picture of their armor and equipment?
Historical figures and any characters from myth or folklore, certainly. Yes. God damn it YES! OCs are awesome but if you can take a historical/mythical character and make them your own, then do it.
And Vikings are certainly allowed. Particularly since they're not as barbaric as many believe. Roman soldiers too, I was even thinking of throwing in a Greek Myrmidon character As for the characters being resurrected, an Einherjar is perfect for this roleplay has they would simply return from Valhalla.
Really, a character just has to meet two requirements:
1) They are highly trained in ancient warfare.
2) They follow some code of conduct, such as a knight's chivalry or a samurai's honor. Romans have a sense of duty so they fit this. Vikings want glory in life, battle, and even death, which is their form of honor. The Myrmidon I mentioned was going to be similar to the Vikings, but instead of fighting gloriously to get into Valhalla, he was going to perform heroic deeds to get into Elysium.