The 12 gauge wire is rated for 20 amps, but that is figured with a safety margin, assuming that the full load would only be 15 amps at any given time. The most that you could do with a subpanel and your existing wire is have 2 - 10 amp breakers in it. That doesn't really provide for much, does it? You mentioned future needs which would require even more ampacity than you presently have. For the long haul it would probably be best to "bite the bullet" and get the appropriate sized feeder to the garage.
See this thread for some related information on feeder sizes and overcurrent protection.