I miss using Niv, it was so nice and simple 
One thing I’ve wondered a bit about is why one couldn’t have a function builtins.isolate (or something) that takes an expression as argument and evaluates it in the more restrictive way, like in Flake outputs? If the argument to isolate is a function that takes an attribute set then isolate could populate the argument from the available free variables, similar to callPackage. Wouldn’t such an isolated expression be cacheable? And you could feed the isolated expression using Niv or whatever.
This is just some idle thought I’ve had, I imagine there are multiple problems with this that I haven’t thought of.