I actually think it's a bad idea, film 7 should be like book 7 and put an end to it all. 7 years at school, 7 books, 7 films. No question.
Actually I think book 7 will be easier than 4 or 5 to make into a 2 hour film, as long as they do justice to the ending I'll be happy.
2 films strings things out too long and I'm not sure there is enough plot in the first part of book 7 to make into a film on its own.
There is also the issue of getting everyone to sign another contract (as mentioned above) and the name (also mentioned above).