The only thing of note Carmella will do in the match will bet to get in the ring, dance around for a few seconds and then probably be eliminated shortly thereafter. If she's in the match for longer than 3 minutes, I'll be surprised. Hell, it wouldn't even surprise me if they went the route of having R-Truth try to come out as #30 for the women's Royal Rumble given how befuddled his character is.
At any rate, I think it's fairly obvious that a good many people hope that Becky Lynch wins the women's RR match and picks to challenge Ronda Rousey for the Raw Women's Championship at WrestleMania. Unless they do a Superstar Shakeup before WrestleMania this year, which is highly unlikely, then Becky winning the RR match and getting to choose who she challenges, just as Asuka did last year, is the only feasible way Ronda vs. Becky goes down for WrestleMania, unless they also go ahead and do away with the brand split altogether before WrestleMania, which is also highly unlikely.
For all anyone knows, even if Becky doesn't win the RR, Vince could just have Becky challenge Ronda Rousey anyway even though they're on different brands. I mean, it's not as if little things like continuity are all that important to the old man these days anyway.