I've been imagining it as something similar to what Diddy and Dixie may have been, kind of a happy medium between a duo like the Ice Climbers and a stance change mechanic like Pyra and Mythra. Pearl takes the lead when aiming to deal in your face heavy damage, Marina takes the lead when it comes time for playing patient defense and set-up. Some attacks might involve them attacking together, others may be following up on the actions of the other. Maybe you can place a speaker onstage as Marina, and then blow it out and create an explosion in that radius with a big scream into the mic as Pearl.
My main desire with any sort of idol character would be that we get a music-centric kit, something that I feel Smash is glaringly missing given how big of a deal music is for Nintendo as a whole. There are plenty of viable characters for that role, but it's a big part of Splatoon and I think they have a good oppotunity right now. So I imagine them creatively utilizing musical equipment (your speakers, keytars, records and turntable...) or abstract soundwave / floating note attacks to some extent, but also rounding it out with their signature weapons. Pearl likes the dualies and Marina likes the splatbrella, and those are both pretty distinct weapons in the context of Smash that go unused by Inkling otherwise. Could play into some charming attacks working off their respective personalities, Pearl forcefully thrusting a rock hand symbol
upward for an up smash or an up aerial, fun stuff like that. With such dynamic personalities and flashy designs the way they fight kind of ends up writing itself.