Ok, so this game is BEFORE the septims rule Tamriel.
What kind of money/currency should there be?

Another type of "septims" or the usual MMO stuff (copper, silver, gold)?
Or something new?

Remans, of course.

Though NPCs usually use the words Septims and Gold interchangeably so it'll probably just be gold.

But isn't Reman Empire gone when the TESO timeline starts?

It'll probably be standard gold coins.

Well yes, but traditions don't die as soon as an empire does. I'm sure Roman coins were still in use after the fall of the Roman Empire.

i really like the 3 or 4 coin systems in most games. i hope they go with something like that