Er, wait, maybe something along these lines would work:
RewriteCond %{HTTP_HOST} ^(nextcloud.domain.tld)$
RewriteRule "^/(.*)" "nextcloud.domain.tld/$1"
If it does, I’d be more inclined to say we could add a checkbox to make an app available on some subdomain name via an automatically created Alias. But I don’t want to hide the implementation and pretend it’s not an Alias and pretend it’s independent of the top-level domain (you can still reach the app on domain.tld/appname or any other ServerAlias that doesn’t get rewritten).