how about returning and stc where the time dimension is the frequency rather than returning time courses or averaging across frequencies? ie tstep param is an fstep.
is it what fieldtrip does?

have a look at compute_source_psd function and the example plot_source_power_spectrum.py

Also I think the spectral estimation should be shared with connectivity module or at least in the semantic of params (eg. mode=fourier, multitaper, (maybe add welch) etc.)