Welcome to tworg! And thank you for the wonderful ideas.

S2TW is not really moddable, rumor has that CA is continuing that trend, i.e. coming out with an excuse there will be no officially supported mod tools etc. IMO, if that is what they end up doing, it is a big mistake in the long run. Another possibility is that they won't be publishing usable modding tools until 24 months into the Rome II. Because first they need the manpower to stabilize the game, second, it give CA the windows of profit, when the revenue is going low, they would turn to the modding to keep the revenue flowing.
But these are just rumors.

I want to mention that the current 8.0 is already running in BI, it looks like an original RTW, but in reality it is using the BI engine and vanilla models, so the Sassanid are already available.

The reason for the 450BC is because the mod's storyline is based on a book I am writing. Sort of a prequel to the current start campaign at 270BC.

What you suggested looks very interesting, if there is a story too, please feel free to share. You would be credited with it of course.