Hmm. . .the thing about Splatoon is that it's "90's cool," with the new stages revealed for Splatoon 2 in particular seeming to channel that. The artstyle has also been brightened up a bit, offering opportunity for old stages to have some new pop.
In that light, Arowana Mall coming back would probably be the best fit for me personally out of potential options for returning stages. It fits the 90's look to a T and the artstyle tweak would make a particularly sharp improvement on it.
Before I come up with ideas myself, I think I'd like to think further on a couple of the suggestions above. The carnival/theme park idea in particular I have one specific thing I'd want to see out of it because it's a recurring detail and that's enough to make me curious: I want it set next to the Ferris wheel seen from Inkopolis Plaza and The Reef.
As far as laser tag goes, perhaps it could be an arena where the lights are darkened slightly and your ink is lit up as though it's under a blacklight? That might be an interesting and unique look. Laser tag arenas tend to be darkened and then given flashy glowing lights everywhere, so it might be cool to channel that. Of course, my idea for it might be too much work. . .
The beach and the snowy location above would both fit the 90's aesthetic in being home to surfers and snowboarders respectively, and they would also make lovely travel destinations in general. Umbrellas and beach towels carelessly left unattended being coated with ink, cozy log cabins with jellyfish wearing winter hats and holding hot cocoa looking out the windows. . .
I want to agree with arcade since that's pretty fresh, but it's also the setting for the local multiplayer lobby so I worry that might mean it's not an option. . .
As far as having a grind rail stage goes, it doesn't have to be entirely grind rails. . .but yes, have a multiplayer stage with grind rails on it. A lot of stages. Many stages. . .at least one stage plz. Have the budget for grind rails, I will you.
As for me brainstorming unique stages. . .hmm. . .would a zoo be wrong? Part of me is curious what on earth Inklings would keep in a zoo, and part of me wonders if they'd keep a zoo in the first place or if they'd allow ink anywhere near the animals if they did. I mean, there's so many different kinds of sapient species in Splatoon, but snails and slugs seem to still be treated as food and pets respectively, so. . .
An airport stage might also be interesting. More specifically -inside- the airport, folks on vacation don't typically go on the tarmac. Maybe luggage conveyors on either side and a snack stand as a vantage point in the middle or something?
