Yeah I agree that if I had more time I could do it... but I also don't really want to, as I usually won't need such material found in detailed settings, and the players are more excited by a true sandbox where they can "make their mark on the world," as opposed to being small players in an established one (even one I've designed).
I don't expect you to have faith in me; we've never met after all. But if I'm starting up a sandbox game, I would start by asking all the players about their characters, some of their backgrounds, family, etc. Then I'm building the sandbox around that, instead of trying to adapt the setting for them.
Nentir Vale is just... fine. I think I find that because it pulls so much material from past other settings it barely registers to me as something distinct, and more of an amalgamation of several other things as the developers tried to create a distilled "pure D&D setting." Which to me just makes it feel boring and unoriginal, but clearly a lot of people do love it.