I'm pretty sure this has already been discussed internally, but I can't find it anywhere on JIRA.
I think it would make a lot of sense if NHibernate were to support .NET Core as target platform in the foreseeable future to enable cross-platform development. I understand that this might be a huge task involving even more aggressive cleanup of legacy code and possibly (probably) some breaking changes, but would go a long way to keep NH relevant. EF 7 will support it (duh), which will definitely give it an edge over NH...
I for one am certainly considering making the switch over to EF 7 for new projects just for that reason, even though NH is still much more powerful and flexible (and I have experience with it).