Gromov's seminal "Pseudo holomorphic curves in symplectic manifolds" (1985) refers 10 or 15 times (for explanations of further applications that he only refers to or sketches briefly and for even "further discussion on $\overline{\partial}_\nu$ for non-regular curves) to his forthcoming "Pseudo holomorphic curves in symplectic manifolds, II", listed as "in press" by Springer.

It never appeared.  Gromov wrote a few later papers on symplectic geometry, but never returned to holomorphic curves.  The paper is the foundation of modern symplectic topology (Floer homology, quantum cohomology, Gromov-Witten theory, symplectic field theory, etc.)