We were discussing the prospect of a Barrens server in World chat (in game) in the past 30 minutes or so and it inspired me to compose this feedback post on the topic for the consideration of the @Innkeeper.
Instead of putting the Barrens (TBC+) server on a PARALLEL track to the Elwynn (Vanilla) server … why not put the Barrens server on a SEQUENTIAL track instead?
The basic idea is that access to the Barrens server is via a Character COPY from Elwynn → Barrens (one way only) … and the eligibility for this Character COPY is available to Level 60 characters on Elwynn server exclusively.
That way, all Players are “required to pay their dues” in the 1-60 play on Elwynn FIRST … and then have the OPTION to copy their Level 60 character(s) into Barrens for the 60-80 run through TBC and WotLK.
Ideally speaking, from a Best Practices perspective, I would make the arrangement a 3x sequential stack, instead of a 2x parallel stack.
- 1-60 Vanilla (Elwynn server)
- 60-70 TBC (Outland server, requires Level 60 copy from Elwynn)
- 70-80 WotLK (Northrend server, requires Level 70 copy from Outland)
By doing a Character COPY, rather than a TRANSFER, through a sequential stack of expansions rather than parallel tracks on different servers … you retain long term community cohesion AND ensure that so long as new Characters keep getting created for Vanilla (Elwynn) that the Vanilla experience “never gets stale” and isn’t in “direct competition” with the other expansions.
- Vanilla is the “gateway” requirement to reach TBC
- TBC is the “gateway” requirement to reach WotLK
Because the character copies (@ 60 and 70) are “one way copies” there aren’t any “backwards compatibility issues” with previous expansions. Likewise, characters that are “copied over” into later expansions “are not lost” from the server that they were leveled on (because: copy, not transfer).
Sequential stacking, rather than parallel play like I’m seeing in the current Barrens server plan would seem to be the Smarter Play™ from a development and management perspective. For one thing, it prevents the Barrens server from “cannibalizing” the population of the Elwynn server (because: drop this for that). It also keeps Vanilla as the starting point for ALL characters, ensuring that Elwynn doesn’t get depopulated thanks to “herd mentality” favoring playing where the highest population numbers are going.
So in summary … I think … that sequential vertical stacking of expansions via character COPY is the superior option to a parallel horizontal stacking of expansions (with a “revert downward via copy” option), structurally speaking.
Using Vanilla to “gatekeep” access to TBC (via Level 60 character copy) helps keep Vanilla evergreen as the place where everyone starts on Stonetavern, while also helping to avoid a variety of backwards compatibility issues via the One Way Ratchet Character Copy @ Max Level formulation for “advancement” into later expansions of WoW.
It also means that Instant Boosts to Max Level go in the 
where they belong. 