I'm still in the super early stages of working on a collaborative sketch/whiteboarding web app. The idea is to have a site wherein users can sketch a diagram/write out some notes, with other users able to view in real-time or save and view later a la Google Docs.

Had the idea a few weeks ago, but with most universities converting to online lectures and many businesses working from home it seems like there's a definite niche in the market for it. Most professors (especially non-STEM) are totally oblivious to anything other than death-by-PowerPoint when it comes to online lectures, and most businesses either snap a pic of a messy whiteboard for posterity or re-draw diagrams through software after they've proven to be worth preserving.