An extension can already make these changes.
I don't think it would make sense to implement a non-standard email field in Flarum just for the extremely niche use case of IPv6 email addresses. We should definitely fix every problem related to the HTTP router, because this impacts users who install Flarum before connecting their domain names, but for everything email-related, I think a community extension would be the best solution.
The same extension would also be able to provide an option to validate IP email addresses by block prefix, something that would be annoying to do (or prone to error) via regular expression.