Re: Characters you'd like to see in future Avengers movies?
Not every incentive, as you said a small percentage. Things like merchandising aren't affected much by relatively small changes in TASM's box office. Little kids (merch drivers) that are excited about Spider-Man merch don't get more excited about Spider-Man merch because he's in Avengers, and if they do, Marvel has Spidey-in-Avengers merch for them already. Marvel can much more cheaply advertise new Spidey merch as opposed to marketing TASM, and giving Sony a sizeable cut - again, agreeing that there is an advantage is not enough to write a contract, they have to agree on the amounts involved.
Anything other than box office draw can be more cheaply made off of Spidey through other avenues, and would be more lucratively invested in the Marvel Studios properties.
Again, since you cut that part of the quote out, agreeing that there is some incentive is not enough, Sony and Disney have to agree to some (hundreds of?) millions of dollars that Disney will give to Sony, and then agree on any strings that might be attached as far as name usage, promised story progressions/involvement/etc. It's just not as simple as: "We'll make more money, so cutting a deal is no problem." Because the money won't seem 'right' to both sides.