I think it would be handy to have the ability to create custom variables for both cross-tenant and tenant-specific cases.
One example of where this could be useful, is if you set a custom “$DEMUserName” and “$DEMPassword” variable for each tenant, then you could have a single cross-tenant deployment of Join AzureAD with the fields set to those variables and maybe a tenant tag to identify AzureAD tenants.
Another thing this could be useful for is to overwrite things that might not usually be changeable. For example, someone mentioned the form factor short names not being changeable right now, but what if you could overwrite them with a custom variable for only the cases that you need it?