If anything, I think that the closest thing to a UEF ace-in-the-hole, if there's going to be one, will likely be the GTC Duke. The ship's crew was completely overtaken by the Vishnans during AoA, and at the very least, they managed to push the subspace drive far beyond its standard operating limits. It's quite conceivable that there's still Vishnan data floating around inside the Duke's computers.
If this is the case, the imagination truly is the limit. Will we see superweapons? Some anti-subspace device? Telepathic hacking?
Never mind, the GTC Duke is the perfect ship for a behind-the-scenes, secret escort mission. Cruisers are weak and vulnerable, and some random heavy fighter squadron could pose a significant threat to it. Unlike escorting a destroyer, the Tevs would not need to discover the UEF's secret mission and mount a full-scale assault, to have a decent shot at taking it out.
Never mind, if they can somehow sneak through the Delta Serpentis node (cloaking device, decoys, diversion, extensive jamming, etc), there's the N-362 Knossos portal. The Vishnans needed a giant ship to alter the node for trans-dimensional transit. A reconfiguration of the portal may be able to accomplish the same thing. They could perhaps open a node through space and
time.
The BP team never fails to impress. Just when I thought it couldn't get any cooler than a mod involving travelling to a parallel universe infested with Shivans, they launched War in Heaven...which was the best gaming experience I've ever had. Whatever they are planning for BP3, I have little doubt that it will be awesome.
