> Maybe a solution could be to internally use a pick_data_type(meg=True, eeg=False) function, which only selects MAG/GRAD/EEG channels.


yes, this might be an option. But we need to check very carefully. The change proposed as is will many tacit 
effects.

So we need to make sure:
- all plot methods work as before (no API change)
- ICA and SSP work as before 
- Covariance estimation will work as before ...
- the TFR functions will works as before and won't include refs in estimation