I've already said: It would obviously be based on results. That's how pretty much every high-paying job works; you get a base salary and the rest is on the results you bring. You could be 'unlucky' and do everything right but you don't get the result you deserve. That's life. Tough titties. etc.
Villa, Sunderland, Wigan etc. wouldn't be able to attract a player if they say "If we win 35 games in the league season, you get £4m. If we don't, you get nothing". But they wouldn't say that because they do want good players. There are ways of designing contracts so that the worse clubs also benefit. Presumably they'd be more attracted to handing out player bonuses to attract better players - so instead of going on the team's performance, they give more money for good individual performers. e.g. Newcastle (not suggesting they're not a good team) say to Ba "Every goal you score, you get an extra x. Every goal the team scores, you get an extra y".
I also think it would be a pretty good way to test a player's character. If he wants to join a 'safe' club that pays out a standard amount with little variation then it says a lot about them. They're not willing to take a risk or they aren't confident of reaching a high standard. e.g. suppose Van Persie is out of contract (

) and Barca offers him £4m a year regardless of what he does, and Real madrid offer him £3m a year (+£100k for every goal he scores). If he thinks he'll score 30 goals, he'll go to Real Madrid...assuming his decision is largely motivated by money - which is the case for many footballers (and most people in general, really).
There are many variations of performance-criteria that can be used in the contract so that each club can benefit. Clubs will want to do it. That's not an issue. Once we're in a world of performance-related pay in football, everything's fine. But the problem is getting there - since no club will want to be the first to do it. I can't think of a rule but there are ways to design shit like this. Happens all the time. Someone will figure it out.
It's not a new idea. I'm sure clubs already do have some sort of performance-related pay. I'd be shocked if Van Persie didn't get some extra pocket money for his form over the past year. But the challenge is now to increase it.