In terms of romantic storyline, yeah there wasn't much left in ROTJ. But it didn't have to be that way. They could have done one of two things to keep the romance plot interesting in ROTJ:

1. Make the romance sub-plot be about Han saying "I love you" to Leia. Yeah, I know he did, but that was really a throwaway line. Han telling Leia "I know" was not just because he felt like it. That was his character. So what I'm saying is the Han/Leia arc could have developed Han into the guy who would say "I love you" back. If you know what I'm trying to say.

2. ANH and ESB set up a love triangle with Han, Leia, and Luke. ROTJ could have finally faced the triangle head.

But neither of these happened. Han's characterization sputtered, and Luke became Leia's brother. So, really, the romance potential in ROTJ was nixed not by ESB, but by Lucas (I think).