There are no plans to add account-specific subdomains on html hosted games as part of this update. These domains are not visible to the end user, so there is no purpose in having vanity URLs.
One potential use case is save-game isolation, is that something you are concerned about?
Only the following domains will be used for HTML games after this migration takes place:
html-classic.itch.zone
html.itch.zone
But we may use other *.itch.zone
domains in the future for different configurations, which is why we recommend those who use site-locks to use the wildcard domain.