Phoenix0001 suggested a plugin like this so Tak (Chaosxk) wrote a short one. Unfortunately, it wasn't properly checking for the amount of players on the server after every map transition. Lux then revised the code but the plugin still didn't work reliably. I then decided to revise his revision and made the "Difficulty Adjustment System" (D.A.S).
The README on GitHub contains all the information you need to know about the Difficulty Adjustment System. If there's something that hasn't been addressed that you're concerned about, feel free to ask on this thread.