The idea I had for them was as incarnations of generative chaos - they revel in creating things. In creating anything, really. Which could be quite destructive on the material plane, I'd imagine, as they wouldn't exactly restrain themselves when it comes to where to make things and what to make them out of. On the other hand, summoning/visiting one would be an excellent idea if you can convince it to make something specific for you.
The Formless Things would ever-changing, shapeshifting monsters. Unlike most shapeshifters, who just assume the complete shape of some other creature, they'd continuously change parts of themselves into just about anything they could imagine. Constants change would be their theme, and they'd probably be more of a physical threat than the other ideas.
Entropic Maws would, suprise suprise, represent Entropy. Despite being Outisders with the Chaotic subtype, they'd actually be Lawful Evil. They'd be patient, polite and follow every law to the letter. To go about their driving goal of destroying everything in existence they would, through perfectly legal means, acquire property and then annihilate it. Tough to stop, particularly for Lawful Good and Lawful Neutral characters, since while they're actively trying to destroy the world piece by piece, they're doing so in a way that's perfectly legal - it's their property, after all. It's the long way to go about it, but they're nothing if not patient. They'd even leave willingly if presented with a law that requires them to do so. They can wait until things change, they have all the time in the world...
I'll write them up for you.