Oozes have more potential than just Blob imitations. Oozes can be creators just as much as destroyers, spawning off new monsters and objects in their wake. One of the most unusual oozes, any the most powerful by far, is the planar ooze. This isn't a monster really, it is a chunk of plane that is animate and is currently active on another plane of existence. This is usually the result of divine intervention, outsider invasion or sapient plane predation. Yes, planes do eat each other from time to time.
The smallest planar oozes are too tiny to be observed with the naked eye, but their impact is just as powerful as the ones larger than solar systems. All planar oozes have an envelope of planar traits, those from the plane they spawned from. The area affected is variable. Some micro oozes can affect a city and others are restricted to a few of their own diameters. Obviously finding and destroying such an ooze poses a very different problem than those that are big as a city. Larger oozes tend to alter the planar traits in much larger areas, but that isn't always true either- some can only affect the area they are enveloping (i.e. their bodies and whatever they ingested). The largest oozes, those the size of galactic clusters, will have traits and powers that extend far beyond their bodies. Whole generations may live on a world, thinking that it is what has always been, at least until they too are consumed.
Some planar oozes are intelligent and others are not. The latter are always directed by something else- a god, devils, angels, elementals, metadimensional aliens, sapient parent planes, etc. This control can be severed, but unless the ooze can be destroyed quickly, that might not be the best course of action. An uncontrolled planar ooze will consume everything it can and grow until it has replaced the invaded plane. And big oozes can grow very fast- whole worlds can be consumed within seconds.
As for what oozes do with what they eat, that has to do with my comment on creators. Planar oozes are in effect, the Genesis device from Star Trek. They convert the existing planar material (and biologicals) into native objects and creatures of their parent plane. A Hellforming ooze would transform all the minor natives of a world into lemures or nupperibos and greater creatures into more powerful devils.
As each ooze is an entity in of itself, with a wide array of powers relating to their parent plane as well as the traits, it is pretty much impossible to provide stats- you have to design your own. Fortunately for the PCs, they usually have an entire plane of allies on their side. Too bad it may not be enough...
