As far as Splatoon features in general go, I think my most wanted feature would be cosmetic clothing. Terraria, Starbound, and Xenoblade Chronicles X have kind of spoiled me. Let me wear one set for the stats and another for looks. The set with the stats can be shown when checking what someone's set is for gameplay reasons, the cosmetic set would be what actually appears on your squid, and people around town can display both when talking to them and looking for something to order.
Considering Splatoon 2's launch offerings are likely solidified and what could reasonably be patched in post-launch considering precedent. . .I think crossover gear would be my most wanted feature. From the sound of it, crossover material won't come from Amiibo and likely won't be in the game at launch, but it being updated in isn't -impossible-.
My mind is thinking back to how the Squid Girl outfit was patched in after a period of time. Some other franchises that would be a good fit with Splatoon getting some gear would be pretty fresh. Of course, considering a very long list of game similarities I do have a first choice in mind that's probably about as hard to guess out of me as figuring out that a sniper is probably gonna try to get onto the middle tower of Blackbelly Skatepark, but of course that's not exactly likely. . .heh. The common choice of people wanting Jet Set Radio gear sounds more than fresh enough for me to want to try it out, though, and if someone does manage to excavate that Mario hat out of a dig site that'd be interesting too.
As great as squids in casual clothing look, going full cosplay squid I'd something I would equally like to be able to do.
. . .Actually, speaking of "casual clothing". . .you know what might look great? A full set of formal wear. That way you could play as a different sort of Agent 4, LOL.
