Super-resolution imaging in vivo
Any superresolution techniques relying on stochastic sampling (e.g. PALM, STORM) is extremely difficult to accomplish in vivo, as the tissue would be moving (e.g. due to respiration) and this would hinder a lot the reconstruction of the image.

Apparently it may be possible to do it with STED but I could not find the related paper*, although the same authors recently published some interesting work on living brain slices.

* Note that the images in that article come from this paper, which is not done in vivo.

Thanks to @vkehayas for pointing out that in vivo STED has been done for instance here Nanoscopy in a living mouse brain