The big issue with the mod visually is giving it a cohesive look. Most of the images of Star Trek races come from screen grabs of the race on the show itself which is of a very low resolution. Getting a high res Ferengi or Federation picture is relatively easy. Getting a high-res picture of a Gorn or a Tholian is much trickier. The Gorn have never appeared in the later high-def shows and a Tholian only appears in a single episode of Enterprise in which there is a visual effect that undercuts the value of the screen grab. The way people have solved this online is to use Star Trek online images (which look pretty bad) or to not use images of the races at all (which undermines immersiveness).
I have chosen to use screenshots from actual episodes when necessary but to stylize all images in a hand-drawn way so that salient differences between the resolution of images is less noticeable.
The best solution would be to have art commissioned specifically for this project of each of the Star Trek races we would need. I have currently created ten major factions - The Federation, Klingons, Cardassians, Breen, Dominion, Romulans, Tholians, Gorn, Ferengi, and Borg each with a Faction photo, logo, background, defined faction file and shipstyle set (with the exception of the Breen, Tholians, and Gorn. The Ferengi shipstyle set is also pretty bad).
I have also made 18 minor races: The Andorians, Bajorans, Betazoids, Bolians, Bynars, Chalnoth, Denobulan, Hirogen, Kazon, Nausicaan, Orion, Takaran, Talarian, Tellarite, Trill, Vulcan, Xindi and Yridians.
Airmaster (Mr. Star Trek) is looking for a more founding-of-the-Federation style mod (wherein the Vulcans, Tellarites and such would be upgraded to Major races and there would be no Cardassians, Breen, Dominion, etc.) and I personally prefer the DS9 Dominion War era. I'm not totally sure how to implement this in practice. Perhaps there will be two different releases for the two different eras or maybe I'll just include some minor races as major races as well. There may some kind of code I can include that will automatically turn on/off major and minor races to reflect the preferred era of the player.
Anyway, that's where it's at. There are likely details of the implementation people will not like (there are implementation details I don't like but had to choose) and I'd be curious to get feedback once the first release is good to go.