Of course, classically, a subdivision of a simplicial complex $K$ is  defined to be a simplicial complex $L$ such that each simplex of $L$ is contained in a simplex of $K$ and each simplex of $K$ is the union of finitely many simplices of $L$, and we can ask for a functorial version.  I don't know of a published analog for simplicial sets. It might be something like a functor $\operatorname{Sd}$ on the category of simplicial sets (maybe required to be induced from a functor from the simplicial category $\Delta$ to itself) together with a natural homeomorphism $\lvert\operatorname{Sd} X\rvert \to \lvert X\rvert$.   Certainly there are known examples.  Segal's paper “[Configuration spaces and iterated loop spaces](https://doi.org/10.1007/BF01390197)” introduces edgewise subdivision, and the first section of the paper “[The cyclotomic trace and algebraic K-theory of spaces](https://doi.org/10.1007/BF01231296)” by Bökstedt, Hsiang, and Madsen defines and exploits a variant of Segal's construction.  Tim, I leave it to you to see whether or not that suits your needs.