No roadmap, unfortunately, at least none that has been made publicly available.
The Zorin brothers work under a "release when ready" policy, which means that there are no deadlines or specific dates for when we could expect any new updates and/or releases, or what will be included in them.
This lack of communication has been discussed before... for me, one of the biggest downsides.
That said, I think the single most significant change will be the version of the desktop environment. Currently, Zorin OS 17 uses Gnome 43, but it's currently at version 47 in its latest release. I would really like to see at least this since it introduces some very much anticipated features: