If the Shivans wouldn't exist in the Freespace universe in the first place, the Ancients would have propably wiped out/enslaved the Terrans and Vasudans.
If the Shivans wouldn't have interfered, then I would see the T-V war lasting for a couple of years more, eventually propably ending in some sort of armstice, seeing as how both the GTA and the PVE, according to the reference bible, were on the brink of internal and economical collapse. But the Terrans would propably gain an upper hand in the conflict for a while, seeing how the had developed the shield system, Avenger cannon and the GTF Valkyrie.
Just a thought, how were the shield systems developed in the first place? Doesn't one mission in Silent Threat point out that the GTI had been aware of the Shivans before the whole thing blew over? Perhaps the GTI captured Shivan shield technology (like in the mission Pandora's Box) at some point?