First of all, it looks great, good job! My question is that why do you limit yourself with only fitness? For example, study sessions are a great example that can benefit from your product.
https://www.youtube.com/watch?v=8tjyfU0pHYc
As you say I'm sure there are many ideas.
It would be nice to be able to do the same thing with a shared Plex server, but not sure if that would be as easy.