In EU its fine, they have connected the PVE and PVP realms into 2 larger groups, just RP is separated. The RP realm does have a few guilds, the people even raid there and I personally dont mind the smaller community there.
The PVE and PVP realms that are connected, have a working community, surely not as big as before, but enough for raiding and pvp. The only thing I noticed is that low level pvp is often not happening, as there are just not enough people during early / very late hours.
My best advice are reroll projects, if they happen in the US. They tend to bring people together, but it can be difficult because of the trading with people outside of these projects. Quite often, the AH is super expensive with people ruling certain trade products and they will always outbid you or buy your stuff and sell it for much more.
This is truly a big downside of rerolling and of course BG´s, as they nullify world pvp, which however, is also dead because of the horde / ally ratio.
Sadly, this wont change on a fresh realm, unless changes are made and I seriously doubt that Blizzard will do these changes.
I am somehow quite confident that they will connect all classic era realms despite realm type or population, maybe even a big merge to 1 PVE, PVP, RP per region and then just tell those who are upset, that they could just play on the new realm, if they don’t want this.
I just dont see them investing time and hours into a classic + system, with significant changes. Too many people want to many different things I am afraid.