There was this DynamicCoin with a very interesting POW approach:
Proof of work
Algo: scrypt
Block reward: scales with network difficulty Max height: 28800 (max 10 days) (after this network will not accept PoW)
https://bitcointalk.org/index.php?topic=500304.0Basicly he made it so that you got always the same amount of coins for your mining power independent of the total mining power of the whole net.
A very interesting concept.
However it seems that this concept lead to multiple forking and consequently self destruction of the Coin before it ever reached the block height of 28800.