That picture looks more like BI than Rome, even in 1.5 the interface doesn´t change with the culture, at least as far as I know.
But if making a new - or even asigning factions to another existing one - really works in 1.5, that would be a major breakthrough. I´ve seen the "culture" tag in some of the txt files, but I thought that was just another redundant entry of a hardcoded function.

Do the nomads in RTW 1.5 have the horde ability? My guess is , but... who knows
I´d not be optimistic on that one, the same way as there´s no shield wall formation or religions in 1.5, I believe those are BI-only features. But the ability of adding or changing cultures would be as much as one could ask for.

I do assume that you´d need new portait folders as well, however, don´t you?