There's a lot to talk about in regards to what comes next for Splatoon. I know DJ gets a bit of talk with the Smash community but I do think Squid Sisters would be a higher priority.It's certainly a possibility. I think it may depend on what Sakurai wants to do with Octoling; we could get unique Octoling or we could get Echo Octoling + someone else unique. Even in the latter scenario, there is other competition such as DJ Octavio, so I'm not banking on playable Squid Sisters, but yeah they're a little slept on as candidates.
edit: sorry for double post
One thing I could see is making the Octolings a unique character and then Callie and Marie get in as separate echo fighters. One for Inkling and another for Octolings.
Though on the side, one thing to consider that is often overlooked is Octolings only get in as an alt for Inklings. There is a part of me that feels echoes are more for totally separate characters with completely different personalities. Octolings don't really feel different enough to Inklings for me. It's still basically the Splatoon avatar so for me it'd almost be like making all the boy/girl characters separate.