The next version of SQL Server is always labeled "vNext", so a tag saying that would be of little use - as Aaron already stated it is also a tag maintenance nightmare.
Since the release cycle for SQL Server is roughly 18 months, you're going to have a "vNext" change about that often. Does anyone know what the codename for 2016 was? It was called SQL Server 2016 (arguably you'll see references to 2015) and at one point was called.... "vNext".
If anything, choose a common value such as the major version number in the CTP build (aka 12, 13, 14) or the estimated release year given the current cadence (2017, 2018, etc). It would be easier to alias a tag of
SQL-Server-2018, should the "name" change, rather than re-using generic tags like