Just like in Fullmetal Alchemist the Movie: Conqueror of Shamballa, somehow Lelouch Vi Bitannia manage to open the gate with the help of C.C. after faking his own death and found himself in Amestris. In the FMA verse the only science well research by the people is chemistry, which is why transmuting a human is hard when all you have to do is manipulate DNA. Will Lelouch set about to build an empire yet again, travel around this new facinating world full of new skills/knowledge to absorb, or maybe just leave a quite life with C.C. Lelouch has mastery on many academic levels so he could also completely transform the way Alchemy works like getting rid of equivalent exchange. How do you want/think this situation would unfold?