No need to increase map size, just path to ride.
When you generate the map you know the distance between the biker and the coin
Also you can count minimum distance for biker to reach the coin.
The ratio between these two distances can be used to manage difficulty.


If the ratio is less than needed you just generate new level. Also you can generate only levels that are theoretically solvable, I mean regenerate levels where the coin is surrounded by barriers

Easy lucky levels are easier to solve with bots and easy to generate (using ratio described)
With target distance there will be far more diffucult for bots to solve the level (I beleive) and player skills will be more valuable

Also why not emission curve like bitcoin has?

I really like your coin concept and want this coin to be successful.