I want to run sandboxes based on Docker images that have Nix pre-installed. (Once the VM boots, apply the project-specific Flake, and then run Docker Compose for databases and other supporting services.) In theory, an easy-to-use, fully isolated dev environment that matches how I normally develop, except inside of a VM.
Nix, on the other hand, solves the problem of building reproducible environments... but making said environments safe for running untrusted code is left as an exercise for the reader.