Also, middleware has to be a node on the outside of the graph. It has to substantially all of a problem, not just most of it.

So a fully functional physics engine, a complete procedural tree generator, a self-contained front-end system, a reasonably complete animation package, that's fine, that's great stuff.

Sixty percent of a solution plus an API is not okay.

A complete solution to a problem (no matter how niche) with the smallest integration surface is key.