Will it be a thing? There are ideas that developers can be certain of and will have no troubles incorporating them into a game. But sometimes the ideas may be risky, not work well in case of THIS game and so on. Would be pretty nice having more sandboxy server where devs would test new content before it is released as a patch or expansion for main servers (a bit like leagues in Path of Exile) and the players who like testing new stuff would be able to do so, as well as perhaps ability to reset their attribute points etc to test out "the best" build for their "proper" char.
I am a fan of the way Everquest's Test Server works, where you can copy a character to it to try out upcoming changes, or just different builds, and there is also a permanent subset of the community that plays there full time and helps tune changes and catch bugs before they make it to Live.
A test server is entirely likely I suspect - this is normal practice with a MMO.
People have gone further and suggested experimental servers - servers where a new ruleset could be tried out without actually introducing it on any live server - and if it works well either add it to the live servers or create a new server with the ruleset. Given what VR has said about how easy they will find it to roll out new servers as needed this seems like a good idea.
People have also suggested separate servers for special events - where the only impact on the live servers would be that rewards (titles or cosmetics for example) could be used there after the special event server runs its course and is discontinued. By special event I am not referring to seasonal or similar events which will assuredly occur from time to time on the live servers, but things like special competitions with rewards for the first characters to hit level 50 or to hit level 50 without ever dying or to explore every point of interest in the game - or find every boss in the game or complete every dungeon in the game or ...well that kind of thing. All without influencing behavior on the live servers. Another excellent idea for the future - not something that would be good to have at release when we all have so many other things to do and no one is bored with grinding or questing yet.
Every couple years this topic comes up again, last one was 2017 (and 2015 before that). I believe there will be a test server, but it’s still too early to discuss specifics.