Well, who would actually cash in for the Million Dollar Championship? You'd feel like Lex Luger at Summerslam 1993. Sure, you won a match, but what did you really win? He's already got Maryse.
I love the idea to cash in at a scheduled time and place. I might even wait until some 3-hour Special. Or maybe have the Miz cash it in on the debut of Friday Night Smackdown on SyFy.
It would be a big moment for the WWE to debut on their new station with a WWE Championship match. They usually make big events like that co-branded anyways, so it could work out.