So far, we have seen the GLA and EU, however, we have yet to meet the third faction. I still think that we can discuss at least the soundtrack for these 2 factions.
With the GLA, I think we're going to see something with less arabic influence. As you can see in this picture,:
The soldier has much more modern armor and looks like a normal freedom fighter of African descent rather than a Taliban. No Taliban clothes either.
So, I think we need to take into account that the GLA are more global and less restricted to the middle east now. Perhaps we might see some American and Chinese GLA cells.
As a result, I believe their soundtrack will sound a bit less arabic, but still very "freedom fighter-esque", and there are at most two or three GLA tracks that sound as such in Generals 1, chief among them being this one:
So expanding on that style seems to be the best choice.
As for EU, they are very technological, similar to the US, but have little to no patriotism attached to them, so an instrumental/orchestral type of electronic or techno music would, in my opinion, fit well. There is one track in C&C Generals: Zero Hour, for the USA, that sounds a bit like that:
Keep in mind the bit from 1:40 to 2:29, or mostly the patriotic cue from 2:19 to 2:29 is too American for the EU, and I was not including that part.
Should the third faction be the USA, their soundtrack should resemble the first game's. But I would like to see them expand on the metallic side, such as we have heard in the main themes of both Generals and Zero Hour:
Though the USA's other theme also sounds amazing:
A remix or remaster of it would be nice.
And the style that was so memorable about China:
Should return, too, if they also do.