We present a synthetic galaxy lightcone specially designed for narrow-band optical photometric surveys. To reduce time-discreteness effects, unlike previous works, we directly include the lightcone construction in the texttt{L-Galaxies} semi-analytic model applied to the subhalo merger trees of the {tt Millennium} simulation. Additionally, we add a model for the nebular emission in star-forming regions, which is crucial for correctly predicting the narrow/medium-band photometry of galaxies. Explicitly, we consider, individually for each galaxy, the contribution of 9 different lines: $rm Ly{alpha}$ (1216AA), Hb (4861AA), Ha (6563AA), {oii} (3727AA, 3729AA), {oiii} (4959AA, 5007AA), $rm [ion{Ne}{III}]$ (3870AA), {oi} (6300AA), $rm [ion{N}{II}]$ (6548AA, 6583AA), and $rm [ion{S}{II}]$ (6717AA, 6731AA). We validate our lightcone by comparing galaxy number counts, angular clustering, and Ha, Hb, {oii} and {oiiiFd} luminosity functions to a compilation of observations. As an application of our mock lightcones, we generate catalogues tailored for J-PLUS, a large optical galaxy survey featuring 5 broad and 7 medium band filters. We study the ability of the survey to correctly identify, with a simple textit{three filter method}, a population of emission-line galaxies at various redshifts. We show that the $4000AA$ break in the spectral energy distribution of galaxies can be misidentified as line emission. However, all significant excess (larger than 0.4 magnitudes) can be correctly and unambiguously attributed to emission line galaxies. Our catalogues are publicly released to facilitate their use in interpreting narrow-band surveys and for quantifying the impact of line emission in broad band photometry.
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