To me, "sports" means games of physical exertion. I don't call Magic etc. sports. I also take the question to be about playing them, not watching them.
Having a game that's enjoyable to watch is probably important as well. Being watchable seems to be a huge draw and I know that games that want to become e-sports are deliberately made in a way that makes just watching the game enjoyable.
This post is a bit of an experiment; Most of the time, Discussion post lay out an idea and this idea then get commented upon. This post, on the other hand, will be purely about discussion on a topic. If this works out well, I'll might post more of these in the future.
On to the meat of this post:
I got this idea from a reddit post on /r/LessWrong.
To quote:
So have at it.
I only ask for one thing and that is to hold off on proposing solutions for 24 hours before giving suggestions for actual sports. In the first 24 hours, please discuss what makes current sports unappealing to rationalists and what aspects a sports designed for aspiring rationalists should have.
Edit: The 24-hour window has closed and solutions and suggestions can now be given.