Runescape's wheel of fortune thingy is pretty intrusive. Buy gold and levels straight from the developer. This exist despite, like you said, one is already paying a monthly fee. But it really isn't the biggest concern. Botters (to such a degree that the market is controlled by them), cheesiness and an overabundance of kids is.
Sadly the sandbox immersive mmo is kind of a niche, you'll always find a low pop.
You could look into the following:
Saga Of Ryzom: No idea if it's being updated anymore, had a few people around last i played, but i think majority are french.
Istaria: Old as dirt, has a few players, most don't adventure anymore as most play dragons, has minor updates.
Trials of Ascension: One i've been keeping an eye on, 3 races(at launch) only one can build, one is a zerk like spider race, standard humans, and gold hoarding dragons. not even into alpha yet though.
Wakfu/Dofus: Cutesie graphics but fun sort of sandbox, has dungeons and quests but they mostly leave you to your own devices.(wakfu is free dofus is p2p)
Another pretty good sandbox mmo that will be coming out soon is Albion Online. It goes into another alpha test Jan 26th. I already got my founders pack. That is the only bad thing. Got to buy into the alpha. And it will have another Alpha in the summer. So they still got alot more stuff to do to the game. But plays pretty fluidly as is.