Is there a URL which can give me the current free hero rotation in a structured form (JSON, XML, …)?
Not as far as I’ve seen, but I do keep track of the information here, where I add the information when it becomes available on “The Nexus Compendium” (can’t add a link, but it should be easy to locate)
And, as a bonus, I’m also using it to try and predict the upcoming weeks Rotations on the site as well.
What do you plan on using it on? 
I found this if it helps :
I use hots complete and im assuming the dev doesnt manually update the free rotation each week, so you could also try reaching out to them
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Bugger, seems like that site is offline for the time being. Looks pretty interesting otherwise.
If anything, I should be able to set up a page that can give the respective data in a more readable manner, depending on the usage.
I’m manually updating my own data, given that I’ve yet to find a way to pull it from somewhere else. Seeing the Collection from outside the game could be fantastic, for instance, now that the Heroic Deals posts/blogs have seemingly stopped.