There seem to be patterns regarding fee required based on time of day and day of week, which is not currently (last I checked) being factored into the fee estimation code. For example, rather than giving a duration, it perhaps ought to give a estimated day of the week or time of the day of anticipated inclusion into a block with a disclaimer that the estimate is based on patterns of the past (which do not predict the future).
In terms of specific development of a GUI wallet, it could be useful to have some visual representation of the minfees that were successful over the last 2 weeks to help the user decide on a suitable fee.
