Probably you need to describe your proposal in more details. I have no idea what do you mean by "target distance".
If the ratio is less than needed you just generate new level.
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.
Ahh, this is tricky because mere existence of a particular distance path to the coin does not necessarily mean that the rider can traverse that path under the constraints of the physics. (Knowing that they could would mean you already have the block solution.) This creates a second situation under which difficulty could pass the threshold of unsolvable, as well.
Also you can generate only levels that are theoretically solvable, I mean regenerate levels where the coin is surrounded by barriers
This should be added to the reference client in any case!
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
Not really. This, too, is really not much different from just increasing the size of the maps. It will hamper the general "broad search" based bots currently in use, but will do little/nothing to hinder more specialized solutions
Also why not emission curve like bitcoin has?
Do you mean why is there not a finite supply? This is potentially a (minor?) concern, particularly for speculators. The intent is that there should never be a case where a user has to either accept a 0 coin block reward, or wait some time to start playing the game.
I really like your coin concept and want this coin to be successful.
If this coin does survive these growing pains I believe it could be huge. It will certainly open some peoples' eyes to the inevitable future of blockchains, in any case!