<shrug>
I think a Rebel shuttle Tyderium is more important than a third faction that shares other ships. Until that happens, I just don't see any potential third faction sharing existing ships. Which makes it an EU only faction. Now, as much as I love the EU, I'm not blind to the fact that there is mixed reception to EU content. I just don't think an EU only faction would justify its existence with sales or gameplay.
I, also, want/wanted a Rebel shuttle Tyderium, but here's the thing; yes, the shuttle's ONLY appearance in the movies (prior to Special Ed) was the Tyderium, BUT it's an IMPERIAL shuttle.
The thing with ships like this is that they aren't really combat ships. The Rebel shuttle happens in the computer games, sure and I'd be fine with seeing it. But the Tyderium's whole mission would just be, "Get this thing we stole back to a capital ship!"
Similarly, third faction ships would just be trying to get their spice to the other side of the board without getting boarded. They are, for the most part, rum runners. The pirates wouldn't be attacking Imperials, and, for the most part, they wouldn't be attacking Rebels. I'm assuming the Rebels would be trying to avoid EVERYONE while avoiding the Empire and that they would be sending out convoys and so forth prepared to fight or avoid the Imperial Fleet, so pirates and rum runners would be no problem for them to handle by either outrunning or outgunning them.
To include the Tyderium or the Fringe guys in this manner would fly in the face of the source material.