Use the API from https://haveibeenpwned.com/ to display the number of breaches of a password, when a user chooses it.
Something like an orange banner "This password is in 4865 breaches."
This should not be blocking (because a lot of passwords are breached already) but it's a nice addition.
There could be a link to the website too, called "read more" or something similar.
This could be done server-side to ensure anonymity or client-side to decrease the load, i'm not sure.