Describe the new feature or enhancement

Using plot_alignment to see where sEEG electrodes are located in the brain. But if you you use any brain surfaces such as surfaces=["pial"] the brain is fully opaque so you can't see where the electrodes are that are inside the brain. So it would be best to give the brain a bit of transparency when you have sEEG electrodes embedded in the brain.

Describe your proposed implementation

As far as I can tell, this is a single line change. I have already created a feature branch in my fork which changes the setting of opacity depending on whether there are sEEG electrodes present for plotting.
https://github.com/kdoelling1919/mne-python/blob/5b039cdfb3da64c89947f09a0bfd83481955f29c/mne/viz/_3d.py#L840

Describe possible alternatives

Another option might be to set transparency generally any time there are sensors to be plotted that are inside the brain volume. Sounds like a bit more work (and more knowledge about the surfaces than I currently have) but it would be more general.

Additional comments

Here is what the plot_alignment looks like in current master version of plot_alignment

Here is what it looks like in my branch.

If people agree this is useful beyond just for myself. I'll make the pull request.

Thank you!