This.But I think crossovers should be kept to a minimum. Give Splatoon it's own identity. Maybe something like a Mario t-shirt would be cool though.
All while having every single one of your abilities and sub-abilities 'Run Speed Up'.Graphic t shirts or snapbacks with other IPs on them would be pretty nice, but that's about it. I personally would love running around in Splatoon with a Sonic shirt on.