I always wanted to do the last emperor of Latin Empire (from the day when his reign starts). Most of (if not all) of your country is of different religion, all of it has different culture, outside of Constantinople and few islands, you own area of Achaia, which has its own king, who will either hate you, or, if you transfer duke of Athens (or Atika, I forgot the name in SWMH) to him, like you, but will be vassal king who is more powerful than you, while his kingdom is not de jure part of your empire. And then there are 3 Greek successors to ERE, who are more powerful than you and have CB for Constantinople. And if you manage to defeat them, I bet Muslims will crush Nicaea and Trebizond, creating new powerful neighbour for your empire.
For more challenging start, you can always make some additional rules for yourself. For example, you can´t borrow money and expell Jews. So, if you borrow something, you have to return it and so on. Other option could be no alliances, so HRE won´t come to help you and so on. Or you could give independence to your major vassals, as I am not sure if Latin Empire actually held any authority over them at this time (they don´t even have those areas in classic CK2) and then try to remake your "empire".