  Highly efficient sources of indistinguishable single photons that can operate
at room temperature would be very beneficial for many applications in quantum
technology. We show that the implementation of such sources is a realistic goal
using solid-state emitters and ultrasmall mode volume cavities. We derive and
analyze an expression for photon indistinguishability that accounts for
relevant detrimental effects, such as plasmon-induced quenching and
pure-dephasing. We then provide the general cavity and emitter conditions
required to achieve efficient indistinguishable photon emission, and also
discuss constraints due to phonon sideband emission. Using these conditions, we
propose that a nanodiamond negatively charged silicon-vacancy center combined
with a plasmonic-Fabry-Perot hybrid cavity is an excellent candidate system.
