Look guys, yes the ending was short but isn't it completely obvious that the ending wasn't id's fault... it was the publisher's fault. After 5/6 years of development time the publisher (Bethesda) forced this game out into the market place for a holiday release.
A September release window that was pushed to an early October window is nowhere near enough to the magical date of 12/25 to be called a holiday release. I don't think Bethesda was putting any more pressure on id than they were already putting on themselves.
I am sure id has some grand epic final boss planned like they always do but it got scrapped. IMO RAGE was epic until the ending but after the credits scrolled I kept saying... I know id didn't have that planned initially.
That's a gut feeling, not a fact. id was under a tremendous amount of pressure at the time - they began hyping Rage way back in 2007, so by late 2011 they were already way off schedule. IMHO Rage's shoe-horned ending was the result of time constraints and mismanagement which spawned when id tripled its staff, joined BGS, and set down a definite release date.
But obviously since I don't work at id, I can't really know.