There's one logo and banner slot, but will think about how we could award more prizes to more people. Would it be so awkward to have different banners for other social media, like Twitter?

Although I'm more than happy to see the activity being generated regarding this contest, I think that from a "brand" point of view it would be good to have the same "art style" being replicated both on Youtube & Twitter - I believe it's much easier for people to recognize the channel by the artwork that they've seen on the Youtube channel (and vice versa).
However when we do hit a particular point in viewership/reach I believe that we could change our artwork from time to time - As long as our logo, whatever it may end up being, stays the same it will allow our channel to be recognized independently of the artwork that may currently be deployed. Perhaps it's a more... balanced scenario?
I agree, its the standard to have a uniform brand theme so it becomes familiar and recognisable
across many social media sites. While the banner and logo size will be different from media
site to media site I guess I'm right in saying any designer will be able to work with the team to resize
his/her design myself included obviously.
I also agree that there are many fantastic designs, Good luck to all.