If Liu Bei had decided to restore the Han under
another force (if he'd have contined to work for Cao Cao for instance) he would have been liable to listen to his "lord", Cao Cao would still have had just as much power over Han and the emperor if Liu was working for Cao, after all, he is only a lower adviser or official.
Liu Bei had to be alone, so that if he managed his dream to restore the Han to the capital, and put the emperor properly on his throne he didn't have anyone else to answer to. It would have been up to Liu himself to see that the emperor was properly restored and not just in name (which is what he was under Cao Cao).
LiuBeiwasGreat is right though, we dont know if he really was ambitious to start his own dynasty in reality, maybe he would have made it back to the capital and called himself emperor, we'll never know because he didn't have a diary