Apart from the problem that by the time they get around to making Deathly Hallows II both Emma Watson and Bonnie Wright will be old enough to be married and pregnant, and some of the older actors may have retired or died, it's actually not a bad idea.
One wonders if, somewhere in the Warner Brothers vaults, complete versions of all the books exist, just waiting for the right moment to re-issue the whole series as 'director's cut' editions. This would, in part explain why Rick Mayle was credited as Peeves for several of the films, even though he did not appear. (IMDB has now removed him from the list of credits). However, his scenes, apparently, were made and must still exist somewhere.
One also thinks of other Hollywood stories. When David Lean made Doctor Zhivago in the 1960s, the story was that the whole mammoth book was turned into a screenplay and filmed, resulting in a film that ran some 24 hours. From that pile of celluloid, the existing film was carved. There is also the story about the 1970s films The Three Musketeers and The Return of the Three Musketeers. It was said that they shot both films as one film and then split the results in two. The controversy was that the actors had been contracted to make one film and had been conned into making two for the price of one.
Whether or not either of these stories is true, it does suggest that there is enough Harry Potter material on film to bring out new versions of the existing films.
Even if there isn't, you can bet that someone in Hollywood is already considering the possibility of bringing out a "new" version of the whole series - complete with every chapter and scene - for television.
We should live so long.
