25-33% is a rather wide margin to call an average. Looking at various sources it's about 20-25% so calling 33% average is padding the numbers a bit. Even still very much of the success rate of onside kicks has to do with the surprise onside kick. When teams are expecting it the percentage is even lower.
So I think what they need is not to increase the chance of regular onside kicks but rather bring back the higher percentage surprise onside kick. Because lets face it, in Madden you can pretty easily predict when an onside kick is coming and if you do it should have the much lower percent chance of a known onside kick as opposed to a surprise one.
At least for online play it should be a very low percentage play IMO. Even lower than real life because anything close to 25% is going to be abused.