Today I'm giving you a quick tip for designing a conspiracy:
You want there to be several named NPC with interesting and often conflicting motivations in the middle management of your conspiracy -- high enough placed that it takes the PCs time and effort to get to them, but not so high that they're the head honcho.
Each middle manager should point to another middle manager, so the PCs experience a feeling of progress disassembling the conspiracy, but in reality they're moving horizontally across it, rather than vertically up it. This keeps you from building conspiracies that grow more and more grandiose as the game goes on.
The middle NPCs should hate each other, or be jealous of each other, or otherwise have reasons to be in conflict. This gives them reasons to be leaving clues pointing at one another. It also lets the PCs pit them against each other and play off their distrust.
Always imply things are much bigger than they seem, so that when you're ready to reach the campaign climax, you can heap a lot of awful trouble on the PCs, reveal the head bad guy, and watch them just barely escape danger and come crashing the gates of the Big Bad.
Also, who says there should only be one conspiracy?