The Ultimate Total War: A Family of My Own.

[Ludens]

I like your ideas as well, but I think TW should keep its historic settings. Also, starting out with stone-age technology and progressing to gunpowder sounds a bit too ambitous to me.

However, the three elements I think most important for the next TW are moddability, a sturdy A.I. able to cope with modifications and (dare I mention it?) a balanced MP campaign.

[HighLord z0b]

I don't know how much of this would be covered in RTW as I only play MTW, but I would love to see more in the way of diplomacy, trade and supply routes for armies.
It would be great if you could cut an enemies supply lines and leave them without food, ammo or reinenforcements. Similarly it would be more interesting when attacking a province if you had to decide whether to try and keep the supply lines intact and move slowly or move quickly beyond your supply lines and either hope that you win the battle and allow them to catch up or pillage and raid the local area for supplies.

The AI diplomacy could be greatly imporved as well, another thing that was done well in Civ.