I think the best legacy characters , the ones that resonate, are those who are introduced slowly and don't seem poised to immediately take the title from the main hero.
Examples: Dick, Donna, Roy and especially Wally.
The characters who tend to get rejected are introduced quickly and also take the main title too quickly, such as Kyle, Yara, and the subsequent Robins who were clearly poised to become the next Robin as soon as they appeared. Jon is a unique case as he had a unique spot in the SUper family until he was (stupidly) aged rapidly just to become the new Superman. I like the character but have hoped he'll be de-aged in whatever the next Earth changing event comes along.