We all know it'll never happen. But if it could, would it level the paying field? Looking at 2 examples in North America:
NFL
Since the salary cap was introduced, this has become one of the most competitive sports leagues in the world. Any one of easily a dozen teams could have won the Superbowl this year, and aside from the Patriots there has been no real dominant team for a long time. The flip side is that equality has to some degree gone hand in hand with mediocrity (Patriots aside).
NHL
A form of cap relating to percantages of overall revenues was introduced a few years ago. So far not much has changed. And the wealthiest NHL team, the Toronto Maple Leafs, were abysmal before the cap anyway. The main result of this cap has been the ability of the team owners to run profitable on a more predictable basis, at the expnse of the players (many of whom are admittedly overpaid despite the cap).
IMO, if it were possible the best solution would be to turn all football clubs back into non-profit social organizations, and forget about any caps. But this is the real world...