I think Crossover maps have a ton of potential if Nintendo be creative
A battle arena from Mario Kart like Wuhu Town from MK7 would fit very well with Splatoon's style of maps.
A Super Mario Sunshine map is almost inevitable if Crossover maps are announced.
Although this is totally based on my Xenoblade fanboyism, I feel that a simplified Colony 6 or Colony 9 would have potential to be a solid map.
If there was a Zelda map, I think Clock Town would be a fantastic choice, but I saw that suggestion elsewhere on the boards, so from my experience with Zelda I would personally choose the Lost Woods, which would be a very gimmicky stage to say the least.
Metroid would be hard to include, and since I've only played Super Metroid I don't have any ideas, but because I still have Metroid Prime Trilogy to start, I reckon there will be lots of stuff there that could be good for Splatoon.
For the Mother series, I love the idea of a Summers map, since it is essentially one big boulevard with a few little streets where houses are based.
Lastly, I think Nintendo should do a 3D maze based on Pac Man, because why not.