A very unlikely but fun idea would be a Playstation vs Xbox Splatfest. It'd be kind of funny to have a battle like that on the Wii U, just to see how people react. It'd be much more fun to decide this way than reading angry gamer comments throwing the most bland of insults at each other.
A fun one around Valentines day would be Single vs Taken. If this were to happen, it'd be fun to see which of the Squid Sisters gets which part. Of course, a more likely Valentine's Day Splatfest would be Chocolate vs. Flowers. But either would be fun.
I still think there needs to be a Mario vs. Zelda Splatfest. It just screams "I'm a good idea everybody wants me to happen just have a Mario vs. Zelda Splatfest already!!"
For Splatoon's one year anniversary, I was thinking there could be a Callie vs. Marie Splatfest, but instead of choosing, you get assigned to a team based on how often you chose their side in all the past Splatfests. I would be on Marie's team if that were to happen: I always choose her team.