Of course, motivation is an important issue in choosing a sport. If you start running, it might be boring and not very satisfying, so it is hard to practice regularly.
But I think from a huge extensive list of sports, a lot of them can be discarded for being too risky (maybe soccer or mixed martial arts?), having no physical/mental health benefits (maybe most e-sports?) etc. So I do not think that "Whatever you can get yourself to do regularly" provides a sufficient condition for finding out whether a sport is rational, even though it is definetely a necessary condition.
e-sports would provide no mental benefit? Here is an article with some studies that disagree: http://www.psychologytoday.com/blog/the-athletes-way/201310/video-gaming-can-increase-brain-size-and-connectivity
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.