It definitely would make sense as a development league. There's a lot of talent that ends up going nowhere, since they can't play college anymore and can't quite make an NFL team. It would help NFL teams that want to hang onto players but have to cut them due to roster size.
I think that they should have it start in early August. Get rid of the NFL preseason (since a big goal of it would be managed via this other league), and have it run so that the playoffs are in December (when there's a big gap with college being mostly on hiatus, and would be over before the big College bowl games or the NFL playoffs start). It might also work during the summer and end towards the start of the NFL season. The key being aim for the downtimes of college/NFL, which would help with people actually watching it.
Of course all of this would be dependent on a partnership with the NFL, which I really don't see happening to be honest. Then again, no one expected the AFL to matter, let alone merge into the NFL.
I think it could be very beneficial for the NFL to have a semi-pro league though. They could get ownership groups (sorta like the Dolphins have now), I know there's plenty of people who would be interested (Mark Cuban, former NFLers like Elway who have tried Arena league, but they would have more support via the NFL).