Anything with monsters. The weirder the better to represent the strange bio-oddities of Gaia's private domains.
Also, the Mechanoid Invasion book could be much fun if you wanted to add more robotic threats to the Machine's arsenal.
Splicers makes a great Phase World supplement. You can easily use the procedure for building a Host Armor to instead build all sorts of bizarre biological alien enemies.
If you don't play Splicers ever
![Sad :-(](./images/smilies/icon_sad.gif)
, the book is still quite useful for sprinkling biological threats or robotic threats into your other Palladium games. Imagine a planet where the Machine won and now looked toward space for future conquest. That would make a cool Phase World meets Terminator campaign.