Hey everyone, I'm going to work on improving Time Frequency Analysis in Source Space as my Google Summer of Code project this year!
I opened this issue, so we can discuss different aspects of the project, e.g. what you would like to have implemented and what not. To get an idea of the project, you can take a look at this google doc, which is a copy of my GSoC proposal. Feel free to comment on the google doc, anyhow, it would probably be better if we keep the general discussion within the thread of this issue.

Problem:

For TFR Analysis in source space, it would be nice to be able to apply MNE's standard repertoire of TFR functions from mne.time_frequency on SourceEstimate data, instead of being restricted to use the source_induced_power and source_band_induced_power functions.

Solution/ alternatives:

There are some possible solutions listed in the proposal.
The basic idea was to enable mne.time_frequency functions to take VectorSourceEstimates and VolVectorSourceEstimates as data input and return one or more SourceTFR classes, which contain time-frequency transformed source level data.
Anyhow, feel free to express your opinion on any solution or alternative (that's what this issue was created for).