I had been discussing almost exactly this idea with a friend over the last couple weeks, wholly unaware of the similar services listed elsewhere in the comments (although I realise there is a chance I heard about one of them in the past but forgot about it). Good job on getting this up and running, looks great so far!

The way we had proposed to deal with funds that were not returned to the user because of failing a goal was to add them to a reward pool of sorts. We would take a percentage of any failed goal funds, but the majority would be given as rewards to others who had completed their goals in a similar area or time period. The details of this hadn't been fleshed out but one potential way to do it we discussed would be to distribute the reward pool at the end of every month to those who had completed gals in that month (proportional to the amount of money staked).

This would potentially remove some people's concerns listed in the comments about the money not being handled well in case of goal failure. Also it might drive more usage of the service if people would maybe even profit.