Here's a couple of events that could be added. Just ideas.
Event 1: Mining Frenzy
Description: Miners seem to be working on overdrive!
What Happens: Miners bring up more resources, but use more power.
Chance: Very small, maybe 10% every 60 minutes.
Event 2: Trader Joe
Description: Trader Joe and his caravan are in town! Stock up now!
What Happens: An Npc appears at the main part of every map with a trading caravan. Players can buy and sell things from this caravan. The caravan would leave after 5-10 minutes.
Chance: Small, maybe 10% every 15-30 minutes.
Event 3: Crab Pods
Description: Several headcrab pods have been launched from an unknown location. Find them!
What Happens: Several headcrab pods spawn on the map, friendly and unfriendly headcrabs spawn around them.
Chance: About 15-20% every 30 minutes.
Event 4: Pilot Down
Description: A combine drop ship has crashed somewhere on the map.
What Happens: A combine drop ship and combine appear on the map. They are both ragdolls. Around the ship, several containers containing resources and items are found.
Chance: About 15-20% every 60 minutes.
All of the chances are suggestions, they could be changed as needed. Leave feedback or changes.