I'm a huge Tool fan, but there are so many reasons it wouldn't work. Not trying to be a Debby Downer, but think about it:
1. As pointed out, their stage show. I'm willing to bet that thing costs hundreds of thousands of dollars per show.
2. They're not metal, or even hard rock, really. If they played most of Undertow then maybe, but Lateralus and 10,000 Days material wouldn't fit the feel of the festival at all.
3. Let's face it - Maynard treats Tool like he used to treat A Perfect Circle - it's a fun side project when he's not doing other stuff. Nothing wrong with that, but it's the truth. There's a good chance he'll be busy touring with Puscifer.
4. Hopefully these guys will be doing a tour of their own with a new album.
5. Ticket prices would go waaaayyy up. I've seen Tool twice, and even their cheap tickets are around $55-$60.
6. The audiences would clash something fierce. While I'll admit, it'd be fun to watch the dichotomy of, say, A7X fans and Tool fans, I feel like Tool doesn't cater to a party/metal crowd. Quite the opposite, usually.
I know they played Ozzfest a few years ago, but I don't see something like that happening again. My money is on Deftones for at least direct support next year. Their new album comes out in October, and their show, while still expensive, isn't anywhere near as costly as Rammstein or Tool's.