...only pay SBD interest on the savings account.

This is a really good idea. While I am against having a large debt obligation for the platform (that increases haircut risk in a downturn) and therefore don't particularly want to encourage HODLing SBDs, this could be an important feature for the long term maturity of the ecosystem.

I'd personally prefer to see SBD function as a stable currency - In other words, it is intended to be spent, not HODL'd. If we want to see real commerce on the platform then we should be looking at the "Velocity of SBD" just like real economies look at the Velocity of Money as an indicator of economic health. In this way we'd require a smaller Circulating Supply and a smaller Debt Level - thus lower Ecosystem Risk.