I actually sent DD a PM with a couple of ideas for alternative tank modules. PM mostly because he's probably already extended and didn't want to encourage too much of a push for new features before the first production run is even out.

But since you mentioned it....
He's already stated that an RBA module is planned, but you are right -- there is great potential to extend the current market (and extend the market for back-up/secondary BB's) with tank modules for dripping atomizers that basically turns the tank into an internal drip shield (ideally with support for 510, i06 and/or 306).
And I'm betting that a "standard 510" module (that would basically be a bridge to a standard 510 connection at the top of the box for use with just about anything currently on the market) would also be popular. Even though that compromises the design intent of the BB it does add flexibility, and I have a tank or two that would look just lovely on top of this!