I imagine they would completely take advantage of their power and give us a horrible ending involving Kaito, Heiji and Akai working together and Ran getting saved by Conan in the last minute while at the same time confessing her love to him. Basically they'd try to make it as fan-girl friendly as possible and make as big sales as possible; so it would have the same type of ending as those movies we've been getting lately.
To be even more of a pessimist, I have a feeling that even if he does live long enough and get to write his own ending there is a bug chance that it would still be a rushed ending and would not give us enough answers. There are just wayyy too many little things that need to be explained that I doubt he has the time to reveal. (Unless he changes his slow writing style of giving us a single important fact every 2 years)