I think they should put a plasma(or energy) SMG in the Broken Steel DLC since there are only two automatic energy weapons in Fallout 3.
Energy Submachine Gun, isn't that a contradiction?
As for automatic energy weapons, I don't think this would be such a possibility compared to the versatility of conventional weapons. And as far as I'm aware there aren't any automatic energy weapons in Fallout 3? Maybe Colonel Autumn's Pistol could be considered automatic, but I'm not sure if it's full-auto or just semi-auto with a high ROF. And the gatling laser 'should' be the only automatic energy weapon in Fallout 3, but it's classed as a big gun...?_?
The problem with full-auto energy weapons is their vulnerability to overheat, so it wouldn't be a possibility beyond the gatling laser or any other multi-barrel rotary system. Energy weapons are meant to be the slow firing hard hitters (plasma at least), and giving them conventional versatility just makes them lose even more distinction within Fallout 3's universal damage system.