Not so much an idea but straight up stolen from football manager. But an addition I think would work well here also.
Every year on football manager you get a bunch of youngsters come through your academy which you have the option of signing. How good they are depends on your youth set up, coaches etc.
Think it would be great if you had a batch of amateurs come through every year (or even 6 months) which you can sign. This could tie in directly with how good your gym is etc. When playing FM this is a big part of the game which I always look forward to.
Something just doesn’t quite work with the current signing of rookies for me. It’s a bit robotic (only need to check every 4 weeks) and way way to easy once you can sign anyone in the world. Not saying totally replace that, but make it a bit harder to sign fighters in that manner and shift attention to your gym (gym probably needs further improving options as well as it gets maxed out quickly).