Bring on the markets for march madness predictions, that should get a good number of people to join
I'm pretty sure this, or any other sports betting, would violate UIGEA (for PI members who are US residents). I suspect that it would also violate sportsbook licensing regulations and/or online gambling laws in some other locations, as well.
There are lots of other sites for sports betting. Introducing this at PredictIt could lead to frozen US bank accounts and to the demise of the site, so I certainly hope that PI won't run that risk.
One exception might be something like a contract on the number of Russian athletes allowed to compete in Rio. At this point, one could reasonably argue that is as much a question of politics as sports. Putin, for one, has been making that argument.