Here's how consumer facing domains were originally meant to work:
- domain.com didn't serve anything necessarily.
- mail.domain.com hosted your email server.
- ftp.domain.com hosted your FTP server.
- irc.domain.com hosted your Internet Relay Chat server.
- www.domain.com hosted your World Wide Web server.
All of these are subdomains. The World Wide Web was expected to be only one of many services you might serve. Nobody imagined it would take over everything and become the only one anyone cares about. Besides Twitter.
Hence, "www." is already a subdomain. So you are asking if you can put your web content on a different subdomain. Of course you can, www is a convention, not a rule.
Now what content do you want to serve on www.domain.com and domain.com? Because as your business grows, you will get users typing that navigationally (i.e into the address bar not the search bar). Is it really better for your business for them to land on a parking page? I am skeptical.
Remember the 'don't move URLs' rule: don't build buzz, depth of crawl and link juice at one set of URLs and then move them. Where do you want your pages to be finally? If you're not ready for that move, if you're staging/testing, you don't need Google yet.
I mean it doesn't hurt if you move before your business takes off, but once underway, stay put.