When I took Geometry in 10th grade (sophomore year of high school) way back in the 1967-68 school year, I think the whole focus of the course was proofs! I struggled so much with them that it was a wonder I learned anything at all! When I did both Geometry basics and High school geometry almost exactly 50 years later last year, I realized that the proofs part of my old course could not have been done in the Khan Academy format of short answer and multiple choice problems. That suited me well because I wanted to learn and re-learn geometric concepts, especially solid/3-dimensional geometry, which my long ago teacher skipped over every time it came up in our book but which I later needed in college math courses.

So I don’t think that Khan Academy will add proof problems in the current framework, although videos and articles on how to do proofs would be a great idea. The other things you listed would be great additions, especially congruence. Two sets of concepts I finally sorted out in my recent journey through Geometry here are:

Congruent vs. similar - I could never remember which one required same side lengths. Now I know it’s congruency.

Complementary vs. supplementary angles - I could never remember which ones add up to 90 degrees (pi/2 radians) and which ones add up to 180 degress (pi radians). A memory device I came up with myself is that the one that comes first in the English alphabet, complementary, applies to the smaller angle, 90 degrees. Anyone who wants to can, please feel free to use this memory device.

Hey, a passerby here, check out this Youtube channel for videos on Geometry - https://www.youtube.com/channel/UCiTjCIT_9EXV1Wp1cY0zaUA . I hope Sal sees cause this channel has nice content.