Interesting news.
Well, never say never I suppose. I think that even if this does not go through, a team-up movie could still be possible.
I think people would pay to see The Avengers team up with the X-Men.
I am aware they are different studios but Disney and WB put aside their differences to make Who framed Roger Rabbit and that worked out to the benefit of both companies.
I'm not entirely sure but doesn't WB have the worldwide ditribution rights for Sony pictures?
I may be wrong but I recall reading something about that or some other two major companies where one has the domestic rights and the other the overseas. I know that the normal practice is to have a Major backing a Minor (In the case of Fox and Fox Searchlight, where Searchlight covers any overseas production companies as an umbrella).
I think if these companies learned to work together, they could all make a lot of money.
If Paramount backs the production with Marvel Studios on helm, they could put in the initial $150M. Sony would then take over distribution rights for the additional $100. Fox can handle marketing between $20M-$50M
Paramount would then get the domestic gross, Sony the international (carrying more risk). Fox would get product rights (toys etc)
Obviously my plan probably has some holes in it, but I think if the studios really sat down, they could come to an agreement that would ensure that they all suffer minimal risk while obtaining a large profit.