Good question! How I imagine it (but it will never be this way) is a combination of RTTS and Player Lock. When you would start a career you would choose between creating a ballplayer or choosing an existing ballplayer. Creating a ballplayer would go as RTTS goes. Choosing an existing ballplayer would leave you at the mercy of the automated player progression. “A”rated ballplayers progress (automatically) faster than “B” rated (so on and so forth). The only difference from Player Lock would really to be able to play any chosen ballplayer with the more fluid home screens of RTTS.
Fringe players, career minor leaguers, rising superstars, going for that 3000th strikeout, etc. The possibilities are endless for the way to play!