The question is not purely scientific but involves also huge amount of creative guess (which only shouldn't violate blatantly contemporary scientific knowledge). I think about something like "The future is wild" or some Dawkins' analyses of probability of evolution of different organs from "The Ancestors Tale". Which different paths were possible for evolution to take and produce different outcomes? Which interesting (cute ;) ) multicellular organism were possible? I think especially about possibilities for Earth life, though if anyone has idea that would be workable on exoplanet (like organism that is reasonable only in case of ex. tidally locked planet) also feel free. Only to narrow down ideas I suggest first assuming carbon chauvinism. Animals, plants, whatever... (they don't even have to be eukaryotes) EDIT: Also partial ideas are welcome (not whole organism, but interesting mechanism that could be used) EDIT2: Example of partial idea: evolving from squids that 'fly' after using jet propulsion, for animals that can glide, to finally animals with ability of true flight.