Next-generation spectroscopic surveys such as the MegaMapper, MUltiplexed Survey Telescope (MUST), MaunaKea Spectroscopic Explorer (MSE), and WideField Spectroscopic Telescope (WST) are foreseen to increase the number of galaxy/quasar redshifts by an order of magnitude, with hundred millions of spectra that will be measured at z > 2. We perform a Fisher matrix analysis for these surveys on the baryonic acoustic oscillation (BAO), the redshift-space distortion (RSD) measurement, the non-Gaussianity (NG) amplitude f(NL), and the total neutrino mass M-nu. For BAO and RSD parameters, these surveys may achieve precision at sub-per cent level (<0.5 per cent), representing an improvement of factor 10 w.r.t. the latest data base. For NG, these surveys may reach an accuracy of sigma(f(NL)) similar to 1. They can also put a tight constraint on M-nu with sigma(M-nu) similar to 0.02 eV if we do joint analysis with Planck and even 0.01 eV if combined with other data. In addition, we introduce a general survey model to derive the cosmic volume and number density of tracers, given instrumental facilities and survey strategy. Using our Fisher formalism, we can explore (continuously) a wide range of survey observational parameters and propose different survey strategies that optimize the cosmological constraints. Fixing the fibre number and survey duration, we show that the best strategy for f(NL) and M-nu measurement is to observe large volumes, despite the noise increase. However, the strategy differs for the apparent magnitude limit. Finally, we prove that increasing the fibre number improves M-nu measurement but not significantly f(NL).