have two interest rates, a lower one for unlocked HBD and a higher one for locked HBD

The model that a number of stakeholders have discussed is to just have the interest rate for liquid HBD be zero and if you lock it up in savings (effectively 3 day staking), you get the specified interest rate (currently 3%, but could be higher, and could be higher still if it weren't paid for all HBD, only locked HBD). I think that is sufficient for a next step without needing the complexity of multiple interest rates. It serves the purpose of giving a reward for buying and holding, and saves some cost paying interest on liquid HBD for people not intending to hold for a significant period (or clueless) who don't care about that interest.

One could also imagine something more complex, with multiple staking periods and interest rates and such, but let's not go overboard dealing with an asset circulating 6 million tokens currently. Small steps, maybe bigger ones later.