A Swift playground is definitely something that we want to add to Swift.org at some point. I know there were some discussions about it already in the SWWG announcement post.
I think it remains to be seen how complicated this would be to achieve but it's definitely not a small task. As there are some things to discuss and consider on the matter, it might be good to start a dedicated discussion thread for this. I encourage anyone who is interested to set one up. 
@0xTim had the idea to link to https://swiftfiddle.com in the meantime, which I think sounds great.