The problem with this plan is that when these guys declare for the draft, and then they go way lower than they expected or sometimes are not even drafted at all, they end up screwed. They have to go through a lower professional league as they have lost the opportunity to go to college and compete. There is no reason this needs to be the case; it's really just the NCAA trying to protect their prestigious image of righteousness. Why shouldn't an athlete be able to declare for the draft, and if things work out and they are selected where they want to be, they can move on. If they drop further than they expected, they can just head back to college, no harm done.
Now, in this world, the big issue that teams are really not protected in this situation. A player could basically choose their situation, and if they didn't like the team, they could go back to school. So, what I propose is that players preselect the range that they must be drafted in to move onto the pros. If you decide that you need to be a Top-5 pick, and you are selected in those first five picks, you are going pro. If you fall beyond that fifth selection, you are no longer in the draft, and you will return to school. Wouldn't it have been better if Adrian Peterson had a couple extra years in the NFL after he showed he was a monster his Freshman year at Oklahoma? NC State's Carlos Rodon probably could have been the #1 pick had he left college after last year, and he is in danger of dropping (only a couple spots, really) because of a less impressive performance this year. There are countless examples of guys who are forced to spend an extra year in school, and it greatly impacts their first contract.
I will admit that there are some areas to figure out, as the NBA and NFL both have large combines where players try to prove themselves. There is no reason that these can't be moved to the weekend, so the athletes can stay in school. Also, I'm not sure who pays for the athletes to travel and stay for these events, but this seems like something the NFL could pay for. If they're really cheap, and they probably are, the cost can be deducted from their rookie contract if they do make it into the league.
Obviously, the NCAA will never let this happen, but the good news is that the NCAA is such an awful organization that they may be extinct in about ten years. If both the colleges and the professional leagues are actually concerned about these student-athletes well-being, there is no reason that they shouldn't work together to provide the best long-term opportunity for them. It just makes sense.