When they announce the Kronos TBC release date, we have a chance to attract a lot of new players who come to prepare (unless they decide to follow a
fresh-start suggestion).
Some marketing and good communication will be needed!!!!!!!!!!!!!!!!!!!!!!
Some of those players may stay and give Vanilla a try too.
One could come up with some semi-crazy suggestion, to make Kronos a heavy-PvP custom server (indeed, reworked ranking, arenas, wPvP, while keeping it x1 for the sake of future TBC transfers), but it would require a lot of work with questionable gain.
Another idea is to permanently make the XP rates higher at some point (closing them transfers of course): old x1 server can only die slowly, while an x5-x100 server can attract decent amount of new players for years.
Other than that and voting suggestion, not much can be done I think. Although good communication is something they need to keep for the sake of the server's longevity.
P.S. But srsly, Kronos is kinda fine. I've just seen 750 players online Alli side, so it's like 1300-1400 peak online. Of course it's Naxx hype, but I'd say there's still a year or even more until the pop becomes unquestionably "low".