Back in November 2018, I felt sure we'd have a "Pikachu vs. Eevee" Splatfest.
For sponsored Splatfests:
1) Almond Joy vs. Mounds (Hershey's) - "Sometimes you feel like a nut, sometimes you don't!" - an age old debate dating back to when they were still being made by a smaller confection company called "Peter Paul".
2) Diet Coke vs. Coke Zero Sugar (Coca-Cola) - their 2 signature diet soft drinks. Diet Coke became so popular in its own right in the early 1980s, that they tried to change the main Coke to match its taste (and failed). Coke Zero Sugar was Coke's attempt to do the reverse, and it worked.
3) Superman vs. Batman (DC Comics) - it's a natural debate...they even did a movie of it! Marvel doesn't really have a similar debate among their most iconic heroes.
4) Squid Sisters vs. Off The Hook (Nintendo) - this, but as a Splatfest tournament. A rematch of Callie vs. Marie first week, then Pearl vs. Marina second week, and the finals pit the best Squid Sister vs. the best of OTH! (personally, I think this will be how July 2019 will go down)
For non-sponsored Splatfests:
1) Soup vs. Salad - the battle of the side dishes!
2) Rock vs. Pop - Come on! Let us in the West have our turn to splat this out!
3) Animation/Anime vs. Live-Action - How to you prefer your visual entertainment?