I mean, I believe the company must own the full copyright of both titles, why not combine the greatness of RTW's mechanics and the greatness of R2's unit models? Was it because of the old Rome engine limit? But this is where the original RTW can use a Remaster.
You don't even need to change appearance of the units in the campaign map, just for the units in battle, that's where us players actually going to enjoy the units' looking anyway. Only the unit model and maybe skeleton if necessary, no need for those fancy animations.
And if the old Rome's graphic processing power really couldn't handle the new generation models, and no easy way to make the game to swap to R2's engine during the battle then swap back after the battle, then stick with R2's unit costume design would be also cost-effective, more importantly the unit appearances in R2 are more realistic(maybe with a exception of Egypt since in RTW this faction was made with fantasy element).