Here’s a snip from the bluepost about Classic Era’s realm connection:
Yeah so the NA PvE East receive free transfers from other PvE realms onto Mankrik while the PvP West gets free transfers onto Whitemane. So (for PvE) if you’re in either Mankrik or any of the other realms in that cluster (Ashkandi, Pagle, Westfall, and Windseeker) you will likely see the largest pool of players for that server type. Similarly, (for PvP) if you’re in Whitemane or any of the other realms in their cluster (Anathema, Arcanite Reaper, Bigglesworth, Blaumeux, Fairbanks, Kurinnaxx, Rattlegore, Smolderweb, and Thunderfury) you’ll likely find the largest group of PvP folks there.
My advice for anyone looking for a place to play is to roll a lv 1 on any prospective realm cluster and talk to the locals to get a feel for what the realm culture is like.