My Level 31-40 Reward Idea
There's been so many 31+ ideas lately I figured I'd have fun and join in.
A mini incense (that stacks with normal so not to reduce sales)
Normal Incense = pink swirled pokemon every 5-6 min
Level 40 = orange swirl that spawns ever 12 min
Level 39 = every 14
Level 38 = every 16
....
Level 31 = every 30
This would be a pokemon ONLY you could catch because of your high level and skill.
They could make the orange swirls little rarer or little higher level too if they want.
Answers
How about just a level floor for all wild pokemon (incense or not) with the probability spread over the other levels through 30. A level 31 player would not find level 1 Mon. A level 32 player would not find level 1 or 2 Mon and so on. The odds of finding a Mon at each higher level would be slightly increased each time.