Hi,
There is no timeline at the moment for an upgrade. We are finishing at the moment the upgrade of other servers in production from 10.0 to 10.1 and from Debian Jessie to Stretch (87% completed), so that is the first blocker to not think about other upgrades (as we cannot support more than 2 versions and operating systems).
We tested internally 10.3, and at the moment of the release we considered it not yet ready for production. We are also looking closely at MySQL 8.0, which also offers CTE support, but has other improvements, such as better metadata handling.
We are likely going to skip over 10.2. However, I recently asked if CTEs were useful for someone at wikitech [0] and I got very few responses, so it is not at the moment a priority. Natural upgrades happen when: No older version host are longer around, the version is tested enough so we are confident it will cause not issues, and we have the resources to proceed with the upgrade. Note that while new features are always nice, bugs normally come with newer major versions, so every DBAs policy is normally wait a few minor versions to avoid being beta-testers.
If I were to guess right now, a possible upgrade may happen mid-next year, tied to Debian Buster upgrade, to either 10.3 or 8.0, but that is just a guess, depends on the above factors. Please let us know how important are things like CTEs or other features are to you so we can prioritize them accordingly.
[0] url: https://lists.wikimedia.org/pipermail/wikitech-l/2018-February/089565.html