How would u have kept the empire at its peak and prevented the barbarian incursions? Personally i wouldnt have just cleanly split the empire like it was. I would have forced some kind of mutual co-operation. I.e. The west helps out and sends military aid to the east in return for economic aid.
Further, Come up with some clear line of sucesion to avoid civil war. Maybye establish two legions under the emperors direct command to act as a detterent to any ambitious generals. (though admitadly they may end up like the pratorian guard)
These r just a few of the top of my head, what about u?