The ideal situation might be something similar to what you have in hockey where there's an independent league that starts at age 16. Those players get stipends, but there are rumors of the best players getting under the table payments. Players are draft eligible at 18, and the NHL team has the right to sign those players until age 21. Some NHL draft picks end up playing in the NCAA, while others stay in junior, until they're ready to make the jump to the pros.
You can adjust the ages to reflect the different physical nature of the NFL, but ultimately it does away with the notion that all players for SEC teams are really "student athletes". If a player wants to play in the NCAA, and wants to work towards a degree he can. If he just wants to devote all of his time to football, and get paid something to do it, it's actually better for all parties involved. The college teams still will get plenty of talented players, and more players that actually want to work towards a degree will have an opportunity for an athletic scholarship.
Basically, it would be a system where the Ryan Tannehill's of the world will play in the NCAA, while Julius Peppers type players could get a little money in their pocket before making the jump to the pros. In the meantime, someone who would otherwise be a borderline Division I player would end up getting a shot at an athletic scholarship, and get an education that he would actually use in the process. At the same time, NFL teams wouldn't have to worry about investing a lot of money in a separate developmental league.
Unfortunately, in order for it to work, you would need a lot of rich guys to come together to build the infrastructure and marketing efforts to get it off the ground. A lot of those rich guys though are already boosters for NCAA programs. The current system has way too much inertia going for it, and, if anything, the rules for pro eligibility only tend to get ratcheted up instead of relaxed.