Here's the deal. A big part of insurgency for a lot of folk is immersion. I think the game's done great in that department by adding local forces and women to Security over original Insurgency. Insurgents tho... They are mostly the same. Including borked English. Barked English, really. It's almost satirical. The insurgents around the world are real people with depth of culture and motivation for fighting. Having them be designer-clothing toting, head-wraping, track-suit sporting, broken-english barking... caricatures... is not as good as it could be.
The most glaring issue is the voice-acting. Not the quality of it, or direction. But mainly the simple fact that they speak in barked English. They really should speak Arabic. It makes sense for the security to speak English since they work with US troops in the game and use US supports. But the insurgents? They are fighting for their home. Their culture. Identity. There's absolutely no reason for them to speak English. They just have to speak Arabic. It would add a ton to the game. With subtitles, playing would be like watching an awesome foreign movie.
The models could also include traditional outfits. (Gun running full body burka lady anyone? :D) And more civilian options, rather than what looks like designer clothes. But that's secondary. The voices are a real deal because they tie the entire basis of the game.
And should Insurgents ever get a Russian voice, that voice should use broken Arabic with Russian expletives like the locals on Security side use broken-ish English with Arabic expletives. That would work great and give the factions character through their relationship with their proxy-war sponsors with the US being more controlling and Russia just using an opportunity to fuck up US.