You can now remove the default domain for each app service where you don't want to use it. Does that cover your needs?

Yes, this has to be a mandatory feature especially in terms of security. The default should just be the ip address or a FQDN.

I think it could be disabled by default and generate a random domain clicking in a button if needed.

Disabling it, or replace it with our own wildcard domain.

O domínio padrão é muito importante e faz uma grande diferença, o que precisa é uma opção apenas para desabilitar depois de adicionar o domínio principal.