Mmmm a few suggestions would probably be....
Mascot for PMC- People would make a lot of funny skins and would base it off of PMC. It'll show their creative side and will have plenty of fun waiting for who would win. EX: A Planet Skin, Dirt Block skin, etc.
Sport? or Hobby?- this would be a fantastic way for other members of PMC to make skin for hobbies that they enjoy! Tennis Player, Baseball Player, Football Player, etc.
Adventure Theme- Make someone who looks like Indiana Jones? They would have a great experience on making lots of unique and exotic skin that they would either see on an adventure, or adventure with. Ex: Indians, Dinosaurs, Gorillas, etc.
Party Theme- Members would be able to make colorful skins that would blow anyone away. They could make clowns to a Robot wearing a party outfit. Ex: Clowns, Bouncy house, etc.
Praise the Doctors Theme- Make Doctor skins to thank all the doctors and nurses out there that help with everyday life situations. Ex: doctor, Surgeon, etc.
Beware, The Elements!- This would be great for some of the really experienced or inexperienced players to make some cool looking fire elemental or earth elemental. People would be making some of the most exotic, unique, fantastic, extraordinary, skins out there.
Feel Free to pick any of these ideas, and sorry if I said a theme that has already taken place in the past.
