I would say anywhere from 5-15% is a good rate to encourage holding. The other thought is a higher rate for the first year, say 20%, on the theory that the first year many coins will be sitting on exchanges as short term trade and rewards the long term investors. Then lower it each year by whatever amount until it is at a rate that rewards holders but doesn't cause so much inflation that the overall value per coin isn't being reduced because of said interest rate. I would guess that there would also be transaction fees rewarded to open wallets that are minting constantly and confirming transactions. I don't how that all works, as code, but it sounds good to me.