Some options spring to mind:

1. All rights reserved - the source is visible but not open.

2. AGPL, to ensure that if someone does take this forward, no part of it can be closed source without your agreement.

3. MIT, which is extremely permissive and effectively you don't care what someone does with it, commercial or not.

Depending on your comfort level I would start with the middle option, I don't think people tend to mind you re-licensing more liberally but if a community has formed and you decide to go commercial further down the road it can cause problems.