Honestly, one other thing I'm looking forward to that isn't new, but that's one of the main draws of the franchise to me, is the return of Splatfests. It's always amazing to play stages at night, and I like how Splatoon 2 introduced Splatfest exclusive stages, so I'm wondering what 3 does with the feature.
Also, I think it's time to tweak how the map rotation system works. If memory serves correct, there have always been two maps per mode. Rotations were once every 3 hours in the original and once every 2 hours in 2. If the map rotation system is kept, then bring down the time to once every hour. Or expand the selection and gives us 3-4 maps per mode per rotation.