Long-running series have two large issues that I feel are somewhat, but not always inevitable. The first is the “Elite Republican Guard” (named after Bill Hicks’ famous Gulf War joke), and the second is “Arkansas vs. The Blimps” (which I named after a Twilight 2000 module).
The Elite Republican Guard involves the antagonists, or antagonist situations, getting less credible as the series goes on. Arkansas vs. The Blimps involves them getting more outlandish. The two are not incompatible.
Coiler's Creative Corner 