As much fun as it would be to publish BTR setting-free and let you use your old Rifts books with it, there are people who would be turned off by a setting-less system. And since the BTR mantra is doing things Better Than Rifts, it would be fun to see if I can extend that to setting design.
While the Rifts setting has a lot of cool things going for it, it conveys very little in terms of consistent theme. Consistency is lacking within the sourcebooks at times, even. Just look at the Vampire Kingdoms sourcebook. It spends quite a bit of space detailing the vampires and the Vampiric Intelligences, then veers off into these small communities in Mexico that don't really support the vampiric theme, and concludes with rules for traveling shows and circuses. Useful material, all told, but it doesn't feel like it belongs in the same book.
Does the Rifts setting have a theme, even if the individual books don't?
Rifts Earth is in the grip of evil forces. Not just the chaos that is to be expected from a setting where all of civilization has been stripped away, but the advancement of supernatural evil. The setting avoids being dark largely by adding layers of gonzo and tech porn. Whether it's the Coalition, the Splugorth, or Vampiric Intelligences, evil abounds.
The only things that stand in the way of this evil are the PCs. The forces of good are small and nowhere near as entrenched as evil. For every Cyber-Knight there are probably 100 demons or undead or other evil minions.
Which brings up an interesting point. If the forces of good are so small, they must be rather powerful to stand up to all that evil. Which brings us to all of the things that Rifts characters can do to themselves in order to stand up to the forces of darkness. Crazies, Juicers and Borgs are some of the most common to appear in the source material. Palladium writer Jason Richards goes into some depth on this.
So we can look at the theme of Rifts as "Sacrificing your humanity in order to protect humans from the inhuman."
The challenge is: If I were to focus on this theme in the BTR setting, it would not be very comfortable to many Rifts players. For them, the charm of the game is that, if you were to list the various character types of a typical party, would sound like the the beginning of a joke where the first sentence ends in "... walk into a bar."
What do you think? Should I design around a dark theme and make a very dark game? Or should I focus on the gonzo element and come up with a setting and theme that enables that?