Is there a altcoin that...?

I've thought for a while now that it would be a good idea to create an altcoin which is PoS but which adjusts its inflations based on the amount of coins that are being staked in order to keep at least say 15% of the coins at stake?

What I mean by this is if there are only 10% being staked then the inflation rate rises higher than it would be at 15% and if it's at 5% it's extremely high etc.

Conversely if 15% of the coins are already at stake then the network will drop the stake rate so that people can put their currency into cold storage without fear of the value of those coins being inflated by much (if at all).

It seems like a simple idea but I don't see anywhere that it is being implemented yet but then again maybe I've overlooked it (easy to do now a days).

Cheers