Unfortunately I don't believe you can do this. The G 14 were on solid ground when they threatened Platini with legal action to prevent action on debt because debt is seen as an acceptable, indeed in many cases the only way, of financing investment and if you introduce a limit on debt tied to turn over you make the same mistake that FFP makes by limiting spending to revenue. In fact, you would take the wealth of the owners out of the equation straight away, which would have meant that in 2008 City would have had the wealthiest owner in world football but only been able to borrow 1.5 times very little! On the other hand United, Liverpool, Arsenal, Barcelona, Real etc etc etc .... while Wigan, Watford, Wolves etc etc etc ? Debt control would have the same devastating effect on competitive balance as FFP. What football requires more than ever is investment not regulation which is vulnerable to endless legal challenge, but it has to be real, planned investment.
On the question of the NFL I'm not too well informed, but instinctively I distrust professional American sport. It concerns me that setting up an NFL franchise is very expensive and few cities seem to have more than one team, indeed some teams have the monopoly of an entire state so the kind o rivalry we get in England isn't actually encouraged or welcomed. Most of the Americans I know - and they're not enough to form a valid opinion sample - watch college football out of preference because of their distaste for the way the sport favours corporatism. And then there's the entire cultural background of the NFL, most notably the draft system which does not and cannot exist here. For better and worse players are assets and we have a transfer market and youth academies here to deal with "distribution", which means they are a major item in a club's financial affairs. Controls on debt or spending mean that the big boys control distribution ever more firmly, but the draft system means that no one of the super rich owners feels hard done by - when they choose their all AMERICAN boys. PL owners would not wish to be limited to English players.