Here, I was using sidekick as a shorthand for any form of established, recurring, and supporting character that is not an antagonist to the main character. But I'm not here to argue exact terminology.
The trouble with Steve is that Diana is set up as being better than Steve in every way in any form of soldier or spying business. It was true with Marston, and it's true today. It didn't matter with Marston, because Steve was the audience surrogate character, but that doesn't work today. So my take on a good modern Steve is to move him away from the badass soldier idea of masculinity, and go in a wholly different direction.
There are of course other ways to write a good superhero love interest, but challenges is a proven and effective one.