105 Di2 - is that coming? And what about 12-speed GRX Di2? And MTB?

Yes, the answer is yes to all of the above...

At least, that is my best estimate. Shimano will be working on Di2 versions of 105, and updates to both the GRX and MTB Di2 groupsets.

As for when... I have no idea. My best guess would be 2022 for an MTB upgrade, and possibly GRX as well.

I don't expect Shimano to release 105 Di2 in 2022, but who knows. They may surprise us all!