One other possible copyleft license, pointed out by /u/zkat@toot.cat, would be the Parity Public License. Like the GPL family, Parity looks to require copyleft-style contributions, but allows contributions to be under a weaker license than Parity itself as long as that license grants the user at least as much privilege as Parity itself does.

Contribute

To contribute software:

• Publish all source code for the software in the preferred form for making changes through a freely accessible distribution system widely used for similar source code so the contributor and others can find and copy it.
• Make sure every part of the source code is available under this license or another license that allows everything this license does, such as the Blue Oak Model License 1.0.0, the Apache License 2.0, the MIT license, or the two-clause BSD license.
• Take these steps within thirty days.
• Note that this license does not allow you to change the license terms for this software. You must follow Notices.

That also has the quite nice advantage of not being as strongly tied to the FSF, which is a quite problematic organization to say the least. Definitely a good option worth considering!