[tnpir4001]

Not really sure that would work any better. I brought in two characters from canon into the first film, and even to my eyes it came across a little awkwardly. Admiral Janeway appearing in Retribution worked a little better because it was little more than a meaningful cameo.

Then there's the complicated backstory. Plus having to explain where this character comes from. Plus the fact that they'd have nothing to do.

Bear in mind the plot of this film is going to be tangled enough as it is. We're going to be traveling back in time not once but twice, and we're going to have two versions of each of the following characters:
1) Gaius Reyf
2) Bradley Prentice
3) Kendra Ronston
4) Braiyon Garr (there will actually be THREE versions of him)
5) Merv Ronston
6) Renee Mitchell
7) Jennifer Hargrove
8) Lesley Kal

Plus Dr. Falwell, Maxwell Garrett, Howard Parks, and maybe a few minor 2378 Fitzgerald crewmembers. At minimum that's 20 speaking roles. Any changes to that cast have to be fully defensible, since I'm having difficulty coming up with enough to keep even this cast engaged for the entire plot. There simply aren't any openings for additional characters here.

Here's what I've got:
A-story: stopping Drakus and repairing the damage to 2399
B-story: Kal and Mitchell / Prentice and Ronston / Garr and Kristie
C-story: the crew from 2399 and the crew from 2378

The established Borg expert and omnidisciplinary scientist here is Dr. Garr from 2378. Period. That was clearly established in Specter when it was discovered that the ISS Voyager had been outfitted with Borg weapons and defenses, and reinforced in Retribution when it was learned that Drakus' physiology was being sustained by Borg technology.

Before suggesting any more cast changes, consider that this is the third film in a trilogy, with a plot that intends to reference events from the first film, and ultimately intends to focus internally on our by-now-well-established principal cast. Add to that the fact that the 2399 version of the Fitzgerald is only going to have seven crewmembers aboard, and there's simply no opening for a one-off character.

Given that, trying to shoehorn in yet another character would be distracting. And as I've said before there's simply nothing for them to do, meaning all the exposition required to explain who they are and why we should care would add up to nothing.