I don't think we should bring up the Emperor...
He hasn't been on that much to even define his personality or how he even thinks. He's made speeches(Could have been easily biased because...duh, he needs to rev up the Nobles and Barons)
I also doubt he would be using Lelouch as a Successor, but the fact he married a Commoner such as Marianne due to her being an excellent Pilot could be a sign that he was trying to aim for a stronger Son -FAILURE, Lelouch can't even dig a hole to save his life(seriously, I could see him dying without food in the island arc cause he failed to dig a hole)
We also have no idea what the King Desires, or what Rangarok/Ragnares or whatever he's searching for. Obviously he wishes to find it first using his Geass Ruins, Clovis himself wished to gain power using C.C. and Schneizel attempted to use the ruins at Kamenija, whoever does Succeed the Emperor will obviously take his place with the 'Ragnarok' plan or whatever.
He only watches Lelouch because he is about the only son(Game chars don't count) Who have received Geass. He's the best candidate to succeed him if he does get beaten, simply due to the fact Lelouch's power and he also assumes his mind is evil(He laughs during Euphie's scene and says " That fool actually did it! "; Somehow I think Lelouch moved up on his book due to it, simply because he didn't see it coming and is proud of his idiotic son killing off his siblings)