Our Season mode in 2018 had the ability to control all teams but we don't have a decision on that for Dynasty at the moment.
We're scoping out how we are going to develop School ratings and what that means to Recruiting. These school ratings (academics, facilities, program prestige etc) could be associated with how well the user performs or even the user influencing what ratings are increased. If that's the case, then controlling multiple teams can get messy.
But we haven't started that part of the game yet - so we'll keep you updated on what the outcome is!