Currently playing in Onyxia TBC and less than a year the realm will be updated to Wotlk and most probably at the end of the cycle the accounts will be migrated to one of the other two Wotlk realms. We already have 3 Wotlk realms which btw are perfectly fine for the 99% of the players. The timeframe of the Onyxia's progress is small - about a year +- 2 months for expansion vs roughly 2 years per expansion originally (back in the time). I am pretty much enjoying TBC and don't want to rush(and don't have much time to rush it anyway).
Generally asking is there any possibility to keep the TBC realm alive? Like a dedicated TBC / Vanilla realm like Lordaeron and Icecrown?
I think a permanent TBC realm has only a chance to survive with significant increased rep, gold, honor and ap rates because TBC is too grindy and that makes it unattractive to create alts and too difficult for newcomers to catch up.
Besides, we had two (medvith and outland) and it failed pretty quickly.
Outland existed for over 3 years and was very populated from P1 until P4. Then after a couple of months in P4 the pop started to decline.
So even though it wasn't possible to keep a permanent TBC realm alive it was still a successful project after all.
In case of Onyxia I think that the content is a bit rushed and having TBC for only a year is too short.
Only 3 months per season is not enough time for the majority of players to gear up.
A permanent progression TBC realm like Frostmourne might make sense. Not one stuck on final patch.
TBC content is built more for progression. Catch up mechanics and variety of content wasn't really established until Wrath. Also Wrath was the peak of WoW popularity so naturally it has a huge fan base.
I think you could in theory create a TBC realm like Wrath although it would be qutie custom possibly moving too far from TBC. Adding a 10 man version and heroic modes to raids. Adding a new catch up dungeon, maybe even take like Blackrock Depths and change it heroic with buffed items. Or just take current heroics and make a buffed version like they're doing on Classic. Add a new vendor with items from past raids. Which I think appeals to players for a one time playthrough, but long term too much custom might fizzle out.
After Onyxia gets to end phase wotlk and playerbase is low, transfer chars to Icecrown, if playerbase is high keep the realm, also add one more going back again from Vanilla-Wotlk,