Well, I don't like your idea
I think most people like buzzerbeater because it's a fast paced game. It's not like some other online sporting simulations where everything takes months to happen. People that don't make playoff or play relegation matches, or lose early in the playoffs are bored waiting for the next season. Prolonging that agony would be cruel and unproductive.
How would you code agents for players on the transfer market? What would the criteria be? How could they choose the teams they want to apply for? And then managers would have to offer them a contract, but they would also want to wait a response from the other team they'd like to join. It would create a huge slowdown and a complicated situation that wouldn't really benefit anyone.
As for the training camp, well you have it, in the form of two scrimmages. Stamina decreases with time, for all players, so the offseason period is a great opportunity to raise it since training stamina isn't connected to the minutes played in a week. And how many times your team lost due to poor free throws? (not really yours SmoOv3, using You as a generalization) Train those.
Offseason as it is, is a time to relax, plan for the future, follow the National Team elections, the promotions and relegations in your league and prepare for a new season
But that's just one opinion
Sincerely yours, Kivan.
P.S. I won't even start talking about the draft because I think it's the most discussed feature on this forums and I already talked about it on the forums so it would be redundant, and would just lead to another off-topic discussion. Suffice to say that I'm ok with the current system