A Steel Cage match makes sense given it would be designed to keep the rest of The Authority out. And then in true WWE fashion, they can have at least one of them find their way inside the ring as usual anyway.
But really, I'd be fine with just a normal Extreme Rules match, or a Falls Count Anywhere match. Doesn't really need to be anything more than that. (What a novel concept - the main event of a PPV called Extreme Rules is just a normal ER match without any other unnecessary bells and whistles to it.)