A music performance simulator. The interface would be a bit like command and conquer, but instead of ordering troops around, you'd be setting up violin sections, and mixing recordings of chorister's voices.
The Nintendo orchestra simulator is a taste, but my dreams were/are far bolder.
Something nice - there is huge crossover in the latin used through much of the choral repertoire. Thus, it's a nicely defined group of samples you would have to record for each singer - would probably only take a couple of hours per singer to get to a point where their voice would slot in and be in effect generic. Perhaps you could break down constonants and 'cambridge vowells' and come up with an even simpler and more flexible sample set.
If money were no object I'd also buy a second-hand stone church, build a house on the back, take up serious lessons and se if I could find a way to get an apprenciceship as an organ builder. Maybe get into choral conducting and try and bring music to people who have talent but haven't had the sort of childhood full of exposure. I'd read a lot of economics. I'd like to think I'd get more exercise.
Oh - I'd also like to work on an interaction engine at the same level as HTML but far more advanced. I think Silverlight and an Adobe project are trying to address this at the moment, but we really need something that's free software and has a lot of the simplicity of HTML without its suckiness (and with better asynchronous event support).
The great thing is that there's so much money washing around the world today that I'm working full time and still getting plenty of time to explore ambitions that are not too far removed from what I've listed above - and what I have written there is my ideal. Computers are stupidly powerful; demand for geeks is high; there are lots of people doing neat stuff with music.