The first is more of a balance issue between the characters. Giving the characters more distinct powers and set strengths and weaknesses would help. But that's more of a worldbuilding issue. Marvel and DC tends to feel like a lap and slap job when it comes to worldbuilding but it's not something they can fix overnight. The soft reboot approach DC favors makes thing worse.
People tend to say shonen battle manga are better than marvel and DC. Worldbuilding is the one issue I'd say that it definitely is.
I agree about the second. It's hard to care about a character that only pops up once a while.
Organizations like Titans Academy, Batman Inc, Avengers Initiative, JLU would keep legacy characters active without putting out more solos, backups, team books. Plus they can be used as a hook for a future solo book and starts to solve the first issue i spoke about.
I preferred the more distinct codenamed over the shared titles. The Signal is a lame choice but I prefer Duke as the Signal than Robin VII. So letting Bart stay as Impulse while Irey is Thunderheart and Jai is Surge works for me.
Ironically Yara has little in story incentive to be Wonder Girl or Wonder Woman so giving her a more unique codename would work better. Plus it pays off her running gag of not wanting to be Wonder Girl.
I like the idea of legacies but often I find them dissapointing or poorly handled.