So, basically you are suggesting 1) a bigger training roster, 2) about twice the games, and 3) the ability to co-op with a friend.
The first two points just make the team more time-consuming to manage if you do it all alone. That's perhaps good for those who can and want to use a lot of time, bad for others. I'm afraid it wouldn't bring more people to the game, as most can't use a lot of time. The third point is interesting, but I think it would be too big a change for the game at this point to be really considered. And to be honest, the farm team concept as such doesn't really bring much to the game, it just makes the teams bigger. So, the role of the farm team would need to be refined a lot.
I think the co-op management idea is good, as long as it brings something new to the table. You could base an entire game on this idea, where you, e.g., have a multi-tier system of teams (main team, farm team, farm's farm, perhaps a junior team as a separate team to these three feeding them all with fresh talent), and the role of the manager could change based on performance. You do well, you get promoted from managing the farm team to the main team. You suck, you get demoted or sacked altogether and need to find a new organization. The way I described this obviously has little room in BB. This is a game where you are the team (you are called the manager, but you are IMO really the team or the team's owner who happens to make all the managerial decisions). If you were the manager, you could switch teams or even be forced to do that. The co-op mode would work better in such a game, I think.