Interesting. Every study I've seen over the past 25 years has recommended sans serif type for online body text, and serif type for headings - with the REVERSE being true for print.

Older users will bug out in a heartbeat if the text is light on a dark background, but offering choice in this is always good. High-contrast options for people with low vision are also good.

A way to add audio at no cost (preferably with the choice of whether to use a good robovoice OR for the writer to record the audio themselves) would also be great.

And I agree that allowing writers too much control over writing and colors will damage one of the most appealing aspects of Medium - consistency and clean design. Writers will love it; readers will hate it. I'm a writer, but I am all about what READERS want. What we want should always come second.