Some websites are providing access as a Tor Onion service, like:

• DuckDuckGo - http://3g2upl4pq6kufc4m.onion/
• Facebook - https://facebookcorewwwi.onion/
• Tor Project - http://bt3ehg7prnlm6tyv.onion/
• Wikileaks - http://gjlng65kwikileax.onion/

Roger Dingledine (original developer of Tor along with Nick Mathewson and Paul Syverson) says something like "Tor Onion services vs HTTPS only websites is like HTTPS vs HTTP few years ago".
-- https://media.ccc.de/v/32c3-7322-tor_onion_services_more_useful_than_you_think#video&t=1041

I think it would be cool to have an option in HTTPS Everywere which instead of replacing http:// with https:// could replace the domain with the equivalent .onion domain.

I submitted a new issue on https-everywhere because I know this program is a collaboration between EFF and The Tor Project and the aim is to making browsing more secure, so I think there could be an interest for this kind of feature.

HTTPS Everywhere is a Firefox, Chrome, and Opera extension that encrypts your communications with many major websites, making your browsing more secure. [...]
HTTPS Everywhere is produced as a collaboration between The Tor Project and the Electronic Frontier Foundation.
-- https://www.eff.org/https-everywhere

Would it makes sense to you? Or do you think it would be better to develop a different project, maybe using the same codebase of https-everywhere?