Regarding the account fee, wouldn't it be possible to make it gradually decrease with the block height?
Well sure, but what would be the goal of that? I don't see why that would be useful.
Since the coin value supposedly will start low and increase with time.
^ I realize that it's a bit distasteful to presuppose that the value will increase. Probably bad omen as well.
That's an assumption, and an assumption which wont hold true, the price of a coin will never grow smoothly over time without ever falling. There is no equation you can use to determine the fee in a reliable manner, the only realistic method is to let miners and/or stakeholders vote on it.