I still believe that it will be a touring festival in a similar vain to Sonisphere
The idea for Sonisphere was first conceived when Stuart Galbraith was working for Live Nation, the organisers of Download Festival. The plan was to hold a series of events that would "translate into other territories". The event did not begin to materialize, however, until Galbraith left Live Nation for the events company Kilimanjaro Live.
Galbraith gained the support of the metal band Metallica, who were claimed to have helped curate the festival for later years; however, rumours were denied of Metallica signing a multi-million pound deal promising the band to headlining the festival for three years.
I believe the idea would be to have the 3 bands, Green Day, Metallica and U2, would be the curators of the festival. 3 very different styles, each playing a different day. The way it works with Sonisphere is that (using these 3 bands as an example) the festival could run in 2 different countries at the same time. For example Green Day could be headlining the opening night in lets just say England, while Metallica headline the opening night in France at the exact same time.
Also it would be silly if this was a single day festival. Fan clashes would be unavoidable. Reading over the Metallica message boards, they have a very negative opinion of Green Day and their fans . . . . .