The idea of redubbing Phasma is nice, but isn’t the bigger issue the fact that the FO captain continues to be dispatched with relative ease? The new character might even exacerbate the issue, since it shows that Phasma probably didn’t escape Starkiller, further diminishing her power.
I don’t think it’s necessary an issue, because I think Finn and Phasma have a complete arc in TFA. Perfect? No, it could use more in the beginning. But I never got the impression Finn vs Phasma was an ongoing fight. For Finn, the fight was more about the First Order in general, and his “moment to stand against the First Order” was against Kylo for Rey.
So come TLJ, I don’t think Finn vs Phasma needs to happen. A shame we lose an interesting character design, but keeping her around as a major player is uneeded.
So this Captain does very little besides have a few likes. I would cut out the battle with Finn, it has poor continuity and ends with a questionable BB8 rescue. Everyone else supposedly dies or is blocked off. It will go like: execution scene + hyperspace ram + a bunch of other scenes + Finn and Rose crashland on Crait.