In reply to nate Hoffelder.

You and me, both, Nate. 🙂

Full sandboxing would be... complicated, due to the nature of PHP. Core does some sandbox-like checks when activating or upgrading plugins, expanding those kinds of tests would go a long way to ensuring a site stays stable. There's some discussion on this core ticket about ways to improve those checks. If you have particular problems you've run into, that'd be a great place to put them.