VasilMichev - Yes! We're excited about the CA part too. Just fyi the "Preview" tag on that will be removed tonight 
TomAafloen - Woooot! Security keys! 
Chris_Clark_Netrix - Yay! Yea haha that was a hard choice before between the old version or the preview version
This should work for Guest Users too and you can apply a Conditional Access policy to protect registration. Feel free to DM me if you run into any issues.
Rusty Brown - Good point. The re-register MFA idea is in our backlog. We do currently have a security info freshness interrupt that checks "Is this info up to date?" but it only works for SSPR. In the meantime you'll have to send them to https://mysignins.microsoft.com/security-info to manage their info.
jurajt - Yep that seems like the right way to do it: using Conditional Access to protect Registration. How are you dealing with remote workers though? Since they can't go to the office for onboarding during this COVID-19 situation.
DanielMisch - I think our dev team worked on a fix for this kind of loop. I'll check and get back to you.