Ask HN: If it were safe to send JavaScript in email, what would you send?

Suspend any concern about security that comes to mind, there are so many. If you could email javascript, what would you email? What could your app email?

I would like to make useful interactive documents, data viz, and such with rich clients, but still decentralized and backwards compatible with any email system.

I cringe to think that such a hypothetical system would become a marketer's playground, even if other forms of security concerns were addressed.

Is this something worth building?