Ooh, Rainbow Brite is a great idea! Have you seen the concept art of her floating around with the astronaut helmet? Apparently she was originally a sort of space-farer! How exciting! It might be fun to try and rework her into a not-frumpy but not-scandalous look! Or do you have another favorite Color Kid you could make? There's some really cute ones!! Like STORMY! :O
For me, I usually just find shows I like and cosplay something from that! You're lucky, the new Star Wars films came out not too long ago - you could be someone from Rogue One, or A New Hope! (Or some well-hidden background character from the originals!) Alternatively, one of the blue doggy aliens from the newest Samurai Jack? Those guys were super cute! Or have you seen the new Tangled sequel, Before Ever After? Some great female designs in that too, it looks like it's gonna be a great lead-in to the new miniseries!
Steven Universe is a fun one, lots of possible backgrounders you could pull from to make an original gem character if you prefer!
Best of luck!