Most of the population is either playing the new areas (Spawns, Eodon, Abyss etc) or crafting/training skills so its very rare to spot other players just running around the "old" lands (on Sonoma at least)
I don't think you will find any free castle spots or luna spots, but there are some super nice Keep spots all over (secluded and even some nice island spots) All depends on what you think you will want to do. Personally I love the 18x18s for the custom designing, but with the new stone crafting you can semi-customize towers/keeps/castles.