This is mostly based on a Athletic Bilbao save from FM23, but as far as I can see there's not really been any changes made since then.
Requesting the ability to either scout specifically for players interested in playing for your B-team, or just the ability to filter away (scouting>players) players who are uninterested, similar to how signing coaches (staff>staff search>include) works.
From my experience whenever you try signing players for your B or C team the players end up having the expectation of playing in the A team, althoug once in a while you'd find players willing to be "surplus to requirements" or "B-Team Player". What I'd like is to find these more easily.
Also, kind of a separate point, but when signing players for B-teams or youth teams for that matter, for the signing not to be registered and counted as first team signings or anything along those lines? I know you won't get fired for signing a bunch of F-rated freebies that the fans seem to be livid about, same as when I sign a 34 year old Model Citizen for the youth teams. Always found the idea that the fans are raving on twitter about a glorified youth coach.