In modern WoW, WM sharding + LfG = gotta kill em all, since there are no Horde WPvP guilds or surefire way to interact with them anyway. Spy addon’s Kill on Sight list is good for killing non randoms tho. However, I have suggested a virtually impossible strategy to hunt enemy guilds. They don’t have to be WPvP guilds, but they could be turned 
There does seem to be an increase in Alliance interest in WPvP guilds and communities, possible further increase with 8.2. Hopefully, if Horde see Alliance being more community orientated, Horde will be more WPvP community minded in response. Are they gonna let Alliance WPvP guilds get away with it!? It’s not the same as the ol’ days, but it can be its own thing in modern WoW! The trouble is finding specific enemies!