How about a mini-series or trilogy instead. It would allow for more time, devoting a specific episode to different aspects of the story.
1) Xavier wiping Magneto's mind, going insane, Juggernaut and Phoenix in X-Men's basement finding out who Onlsaught is, etc.
2) Onslaught rising to power, the first wave of destruction and the assembling of heroes
3) Fighting, scrounging for equipment, Franklin Richards being abducted, and the Onslaught Impact, like Wolverine putting out fires, Spider-Man encountering sentinels etc.
4) Big offensive, heroes get smacked down, Hulk gets seperated from Banner, more stuff
5) Thor and the rest plunge into Onslaught and defeat him
It's been awhile since I read the Onslaught stuff, and I don't have most of my Onslaught comics anymore, :'(. So i might have some of this stuff wrong, forgive me in that case. And in that light, does anyone know where I could find some TPB that deal with it? If they're still available.