A friend suggested searching for unclassifiable animals, which seemed like an excellent idea ...
These two critters might be related to the (believed to be) extinct Ediocara.
Here are some oddities.
Rock rats required a new family of rodents.
Barreleye fish have a mostly transparent head surrounding eyes that can swivel to track prey, and the fish can swim straight upwards.
The eastern emerald elysia is a sea slug that eats algae, then incorporates their chlorophyll into its body. Yeah, I'm going to call that a good strong candidate, because plant-animal composites can do some really powerful things, and it also has the very rare stunt of cross-species DNA transfers. It wouldn't take much tweaking for that to become a stupendous method of adaptation. :D
The number of species in the ocean is unknown, and indeed, around 91% have not been found and classified. On land it's about 86% unknown. So great, I'm probably looking for something that isn't even listed. It's going to be obscure and probably small. Also talking about freakish things can kill your career, which is not exactly a great incentive to talk them up. Given that oceans cover most of the planet's surface, statistical probability indicates this is a likely location for the next great leap of evolution. Well, drat.
See also cryptids. Strip off the rather unscientific stigma against organisms which are rumored but not proven to exist, and all sorts of interesting possibilities emerge. While famous cryptids tend to be large and dramatic, not all of them are, and some examples of subsequently proven critters have clever traits believed to be impossible. You don't want to be the stupid white guy who runs screaming out of the bush, "The cobra spit at us! The cobra spit at us!" while the brown people laugh their asses off because they've only been warning people about it forever. Logically if a critter developed some really new trait -- like invisibility, teleportation, scent-masking, etc. -- then it would seem unbelievable. Imagine how startling chromatophores or body heat must have been once upon a time.
Science is all about admitting when you don't know jack. But the history of science can sometimes tell us roughly where to look.
Does anyone else know of unclassifiable or otherwise unusual critters, preferably of recent advent, that might be poised to take over the world after humans finish pounding the biosphere to paste?