I approve of this turn of events. It was shown in SS that he is somewhat of an inventor, so perhaps he teamed up with the Shroobs and built up an army of mind-controlled monsters to take over the Mushroom Kingdom. :3

The Fawful helmets and the green Shroom UFO's also make me think Fawful has some level of control over the Shroobs and perhaps some kind of mind-control technology. Fawful might be pulling a Dark Samus here...