In most of the reports I design, I would like the date to be filtered as YTD (or other relative filter), but also give the user the ability to change to a custom start and end date. The workaround for this that I have been using is to have a parameter that toggles between the relative filter and the date range filter. This gets a little confusing to the user since the date range filter doesn't always correspond to the actual dates in the relative filter, but they are both visible.
What I would love is a way to default a parameter based on a formula. This way I can simply put a Start and End Date parameter, have start date to default to current year start, and end date default to today (or prior business day)