These global events I think are key to keep things fresh. They just need background notifications to alert players who are in a specific area. The global events don't have to be a big new Pokemon monster but that would definitely be fun to show all the trainers who are currently battling the monster. There should be a time limit on when you have to defeat the monster to earn the prize which encourages people to get out there.

There could also be fun challenges like "Magikarp invasion!" - 400 Magikarp have invaded Fisherman's Wharf. They need to all be caught in the next hour or else they'll threaten native species.

Create some fun quests and challenges and randomize where they spawn and make them time limited.