I really don't see why they couldn't have just stated that Zwei is German, Tira is Romanian (for example - plus it fits her gymnastic/acrobatic style) and Zas is ......Babylonian? Plus, pre-SCIV, the Dev team were really clever and creative about adding fighters from new regions with each iteration of the series - from Okinawa, to France to the Philippines etc. This seems to have stalled, and lead to a slew of characters from "Unknown" backgrounds. I'll say it again, "Unknown" is not mysterious - it's lazy.
Exactly, I really don't see the appeal of that either.
It really is simple to create a character. Just create a character from a different country, and give them a personality.
Next is to research on any native inspiration for their attire, as they have been doing. Hence Talim.
Next is their weapon, and there are countless lists of weapons from various countries. If already taken, just make something original, as they have been doing from the likes of Setsuka, Talim, and Hilde. No one is bragging about the weapons of the character. Just where they are from. They can create an original weapon, but add a native twist to it.
Once they are done, just think of how they could be put into the story. Protagonist, Antagonist, Neutral, or......Pyrrha's role in the story.
Just doing that could create some extent of originality within the series. It's just something that needs to be explored more. Arabic/Persian is one of the easiest examples, as it has never been done before in the series, and it includes a group constantly in conflict with the southwest side of Europe during the historic times.
Soul Calibur is supposed to be "fictional, but based on a historical setting." Soul Blade/Edge even tried to tie characters with history/myths (e.g. Mitsurugi formally being a part of a historical clan, Rock migrating from Great Britain to the New World and encountering Native Americans, Sophitia and her encounters with Olympians, Korea in conflict with Japan, Mentions of the "Wokou" on Li Long's profile, etc.)
While characters like Tira being unknown could be understandable at the whole "orphan" thing, she could at least know when her birthday was, unless she has a birthday for each of her personalities or something.