I'm looking to create a sidechain on Stratis as well, maybe a USD pegged stable coin or a gold/silver backed one? Smart contracts can definitely make the process decentralized so no need for banks or third parties involved.
Still I haven't decided but somewhen in spring or summer I will surely be working on a project.
The sidechain itself couldn't give you a stable coin since the rate of exchange between Strat <-> sidechain coin is fixed. However, as you say, it would be possible to do this with a smart contract on top of a sidechain. There are a few models you can look at to give you some idea of currently working solutions, i.e. Dai from MakerDAO.
My one piece of advice would be to see how you would feel about using an existing sidechain, such as Cirrus (federation members are currently Stratis Group Ltd, so if that puts you off, then we can work towards getting a sidechain with a federation run by broader community members), simply because using a sidechain shared by a number of token contracts will offer easy access to the network effect: you can piggyback off the success of other tokens etc.
Crypto-collateralised stable coins are the way to go, IMHO. It would be valuable for Stratis to have one.
