The two license suggestion is a good idea. As for the version stuff, I agree with that also. I believe that was already discussed, so perhaps that will be the case for code plugins. It is unlikely however, that people will stay on previous versions of the engine going forward. They will begin to expect that plugins will just work and that developers will have their code functional upon release of new version. is especially the case because the engine is available early via github and in preview form. Customers are unlikely to cut developers slack for not being up to date and running on new versions of unreal.
Imagine the case where a plugin manages data in a map. Unreal releases a new version of the engine(it has gone through git phase and preview and is now release). Customer wants to move onto new version and get great new features of the new engine but they used your plugin for their project and when they load their project, the plugin doesn’t work and they lose data, forcing them to stay on the previous version. They will not blame Epic, they will blame the developer of the plugin for not keeping up to date.