For realisms' sake, the Elephants should have a speed of 4. This speed makes it less vulnerable to agile early game archers, and especially the Mongolian horse archer. This makes it less vulnerable to conversion because it can destroy the healer, priest, or missionary before it gets in range, or if it missed. It will be able to catch up on infantry or follow in line with them without creating as much traffic. They will be also unable to move into wagons. They will also be a viable unit for and destroying siege weapons.
This will make players fear elephants more, and make more converting units, and their own elephants which almost nobody uses. I only did once to push back heavy amounts of infantry where catapults were not available.
It actually did work out well.
Have any other ideas please share!