I think the whole field of anabelian geometry fits the bill, even if it's perhaps more focused on going the other way around (i.e. applying homotopy theory to number theory). Anabelian geometry is a 'program' launched by Grothendieck in his famous Esquisse d'un Programme, and is all about translating arithmetic geometric problems to problems in homotopy theory.
As an example of a specific instance of the anabelian philosophy, we have Grothendieck's celebrated 'section conjecture', which states (in one form) that for a 'nice' curve $X$ over a number field $F$, the rational points are in bijection with the sections of the exact sequence \begin{equation} 1 \rightarrow \pi_1(X_{\bar{F}}) \rightarrow \pi_1(X) \rightarrow G_F \rightarrow 1 \end{equation} where $G_F$ is the absolute Galois group of $F$ and $\pi_1$ is the algebraic (etale) fundamental group. In case the curve is over the complex numbers, the etale $\pi_1$ is the profinite completion of the regular fundamental group, so there is a very close connection to the classical stuff of Hatcher. The conjecture is still a wide open problem, but any proof would mean you could check something of number theoretic interest (existence of rational points on curves) by studying maps between certain generalized homotopy groups!