Re: So what will be this Preventable Event?
Sentinels could prove too costly, especially with such a large and expensive cast to pay. Bringing back big names like Jackman is going to cost money. Hugh is said to have earned $25million from X-Men Origins: Wolverine - those sorts of silly salaries don't leave much for the actual film itself!
If not Sentinels, it could be some other terrible future. The Legacy Virus. Genosha. etc.
The one problem with an opening scene showing the X-Men running from Sentinels is that it will be a repeat of the Danger Room sequence from The Last Stand. Won't audiences remember that?
I don't think it will be anything 'obvious'. Not if Bryan Singer has come up with another super-smart story idea.