First off, this mod is going to be great. The next best Stargate mod I've had been privileged to play was the one for Starwars: Empire At War, but of course as awesome as that engine is, it is still very limited because the focus was spread between space combat, ground combat, and a global turn-based RISK-styled interface. The Sins engine, with its more focused agenda, is going to set a new standard in RTS design, I have no doubt, and as the conspicuous representative paragon of complexity and elegance (a difficult juxtapositional ambivalence indeed), it will be the modding platform of choice for years to come.
That being said, this mod has
a lot to live up to. The pressure is on and all eyes are focused on this project. It doesn't have the luxury of obscurity because of how big Sins is becoming, so don't F it up.

Now, as this is the first Stargate mod, it is safe to assume it may become THE Stargate mod for Sins, and as such, it needs to be done right. I'm speaking from the perspective of an rabid Stargate aficionado, with over a hundred gigs of the movie, both series (SG1 and Atlantis), as well as a screener copy of the Ark of Truth movie months ahead of time (yes, I bought the DVD as well as soon as it hit the shelves, support the franchise people!), and cosplay as SG1 at various SciFi conventions (Yes, I am a
Level 12 geek, tremble before me.) So when I say it needs to be done "right", please don't interpret that to be indicative of any pretension or supercilious proselytism on my part, but rather, I wish you only the best, and I hope that this project can realize its true potential, being everything the fans deserve, and more.
It is incumbent on anyone who would take it upon themselves the responsibility of painting with a different brush -- in this case the medium of game modification -- the epic which is this generation's Star Trek, our Star Wars, to do so with as vivid a color palate as necessary to persevere the fidelity of the very material that is the subject of your fastidious affection, but also to exercise copious, but
consistent artistic license through interpolation where necessary in areas where the camera's eye has not yet explored.
The Stargate campaign setting is vast, verbose, sometimes seemingly extemporaneous, though always ultimately self-consistent, but most importantly, unified and singular. Atlantis is not merely a "spin-off" series, such as the relationship between
"Cheers" and
"Fraiser", or for those of you old enough,
"Dallas" and
"Knots Landing". On the contrary, the relationship can better be described as a singular, shared continuity with routine cross-over and homogeneous concerns served up as the plot du jour, garnished with communal MacGuffins arranged elegantly between both stories. This lack of canonical segregation is the keystone in the bridge that connects both programs in a shared story universe and must also be respectfully observed and maintained in any canonical recreation vis-à-vis alternative medium. Any mod that attempts to tell only one side of the story will be incomplete and ultimately exclusive to either one group of fans or the other.
Admittedly this will be quite an undertaking, but nothing worth having ever comes easily. Many new roles will necessarily require original content while observing the unique idiosyncratic stylizations of each race, but this is by no means impossible. In fact it could prove to be some of the most fun for the design process and quite a treat for the fans to see sides of the campaign setting that are completely new to them.
For example, I offer the following: The Goa'uld "Mastaba" Class Battleship, just as one possible new-content Super-Capital. I take no credit for this image, I don't even remember where I found it (most likely a random post on 4chan), but I obviously liked the image enough to save it. Regardless, the design shows respect for the established Goa'uld stylization, but represents a logical escalation in a possible off-camera arms race; a ship that very well could have existed off camera. That kind of thinking will be required to flesh out all the roles expressed in Sins to complete this project.

Now on the matter of specific role assignment, I recall it being suggested that the Wraith might be used as a pirate race? I think that, given as they are as the Goa'uld to Atlantis, they play too large a role to be demoted to such a small appearance, and should be a full fledged selectable race. However, I feel whom would make a perfect candidate for that role would be the Genii Confederation. Additionally, the Lucian Alliance could play as an SG1 alternative on different maps.
I'd very much like to help with this mod in any way I can, but I unfortunately lack any artistic or programming experience. However, I have a nearly encyclopedic knowledge of the Stargate campaign setting, thus my assistance would be limited to an advisory position, but one which I hope I could make a valuable contribution.