I can't think of a single character Marvel has that has more value than the Spider-Man franchise.
Character vs a franchise. Franchise, I would think the Avengers franchise may be worth more than the Spider-man franchise at this point. Character, yeah you are right when you account merchandise and everything else then no single character is worth more than Spider-man.
Nothing says it has to come from Marvel - it could come from Disney (most likely, though, it would come from Fox). Also, nothing says it has to be a single franchise. They could transfer multiple franchises in exchange for Spider-man. If the new owners aren't particularly interested in trying to compete in the world of Superhero movies, a deal could potentially be reached.
Though personally, I bet the likelier outcome of a less interested new owner is that they make a new licensing agreement to let Disney/Marvel do whatever they want with the character in exchange for a permanent payday that they don't actually have to work for.
There has only been a few times where Disney gave up IP ownership that I can think of, i.e. selling Miramax or the Power Rangers.
There are possibilities such as Netflix, Amazon, or Apple buying SPE and since they aren't interested in theatrical releases they would either A) Have to change their ethos to release theatrically, B) Continue not releasing theatrically and lose the license or C) Sell the license at fair price and get something in return. I am personally very bullish that the above companies, especially Apple, needs to acquire a content provider and library for their streaming service. Someone like SPE, CBSViacom, MGM, Lionsgate.
Other possibility includes CBSViacom merging with SPE since they want to scale and have their own streaming service. Difference here is that they will release theatrically, so the hope for Disney is that the new proforma company would want to deleverage and focus on all the other properties they own, which includes merch rights. A fair price assessment is usually done by investment and financial service companies such as JPM, Goldmans Sachs, Stanley, etc
The problem is not that Sony won't sell the license and/or rights to previous Spider-man work, the problem is that they won't sell it at a fair price. The license is worth a whole lot more to them than it is to Disney, hence the impasse. SPE being bought by one of the above
may (emphasis on may) open the possibility to Disney getting the property back at a fair price.