Stuff that should've already been in the first game:
1. Voice chat.
2. A Rank system that doesn't punish you for having a bad internet connection.
3. Better gorramn servers already for Christ's sake get on that Nintendo.
Stuff that's totally sequel material:
1. Playable Octolings.
2. Different hairstyles.
3. New gameplay features made possible only through the next console.
4. Moar Single-Player Campaign.
Stuff that's not gonna happen:
A side-quest where you build a relationship with Annie, similar to the side-quest with Peatrice (the Item Check clerk) from Skyward Sword.
Also, what if, instead of clothes being randomly picked for availability each day, they're all available from the start, but they're crazy expensive and instead are randomly picked to go on sale each day. Think like how it is now, but in addition to the clothes you can buy at their current prices, you can also buy the rest of the clothes at extremely marked-up prices. I don't actually know if this'd be a good idea or not, just kinda throwing it out there.
Also, also... To the people who aren't ready to talk Spla2n just yet, it may come sooner than you think. Nintendo's probably going to want it as an early release title for their next home console (whether that's the NX or not). The only alternative I can think of is if Splatoon gets a big console exclusive update and we get Splatoon 1.5 before Spla2n.
Actually, there's a thought: What if the next home console is backwards compatible, and instead of immediately coming out with a sequel to Splatoon, there's a big patch that adds all these features to Splatoon that are only possible through the new console? Do you think Nintendo would do this, and if they did, would you be upset, pleased, or hungry?