Hi so I been seeing many people getting stuck on fastboot after flashing ROM so here is the method u can use if u can't boot to TWRP also.
I have tired on PA ROM and it works so idk about others ROMs ..
fastboot flash dtbo dtbo.img
fastboot flash logo logo.img
fastboot flash vbmeta vbmeta.img
fastboot flash boot boot.img
fastboot flash misc misc.img (u can get on fastboot rom not .zip)
fastboot flash recovery recovery.img (recommend used twrp-3.3.1-2-davinci-fix).. Russian one
And after those comands do run TWRP auto installer.
U can find those files from fastboot stock ROM. This will save ur data and u won't needed to flash Stock ROM ..
Hope I explained everything if anything else needed comment down below.
Dude, your guide is a life saver! Spent 5+ hours after stuck in bootloop during my PA10 install, ended up losing my TWRP (with NO OS installed!) and got stuck in fastboot mode. Thought I bricked my phone for good until I followed your step EXACTLY, and voila! TWRP is back! Now I can install MIUI 10 back into the phone. Tried using MiFlash, got antirollback check error (maybe due to the fact that I installed MIUI 11 beta before I went with PA 10). Thank you again man!