U2F Security Key (YubiKey) support
I suggest supporting U2F because security keys are the most secure method for 2FA. Unlike TOTP codes generated by an authenticator app, U2F keys are phishing-proof. They only work once you have registered them with a website.
In my opinion, it is worth improving NextDNS account security because:
1) Hacker who takes over NextDNS account will know user operating system, antivirus, online services, etc. This information will significantly help the hacker to prepare and conduct a successful attack.
2) Hacker can use the rewrites feature (in settings) to redirect the user to a malicious website.
For the same reason (to improve account security), I also suggest voting for Tim Schuckelt idea:
"It would be nice if you could enable the option to receive an email alert on successful login."
This idea is about an option, which can be enabled/disabled depends on your preferences.