World Boxing Manager really is good, but the dev is still working on "On the Western Front" so I doubt he'll be getting back into boxing soon. He's one of my favorite solo devs, doesn't seem like a great programmer yet comes up with these ambitious ideas and completes them, bug free, using sheer determination. I think he's been working on Western Front for over 2 years now. I doubt he makes much money doing game development, but he deserves to. I hope there's a hit in his future.
By the way, WCBM is different and I don't think that's what you all mean. World Championship Boxing Manager is a game from the early 90's which has been adapted somehow to run on Steam. It's also a $5 boxing game on Steam so I don't want people to mix the two up. World Boxing Manager is where it's at.
I'll second that Leather is an excellent mobile title. I've dropped quite a few hours into that.
Looking forward to seeing what you come up with, Derek. I agree that it's tough to add the role of promoter, unless the deal is that you're managing a venue or something. Boxing is just too different, with fighters competing for different organizations at once, and the fact that you don't really sign or develop boxers if you're running an org. Gym owner/Coach seems like the only way to go IMO.