I'd get a nice camper van shell, put in an inverter, then use home audio gear, HDTV, computer, BluRay, mobile satellite antenna with receiver, high speed wireless, etc. (add anything else you can think of) and basically just pack a living room in the back. Dolby 7.1, THX, reverb, EQ, any sort of signal processing out there would be part of the deal.
Up front I'd have those Fantastic Sony Xplodes with a WalMart special HU just to keep my oar in car audio...LOL!
Oh okay, enough funny stuff. What I'd like to do with my Cadillac would be to find someone who knew how to deal with the door panels, which means not having me wind up with all my electrical going dead. That wiring doesn't like to be messed with. If I could find a place that restored Cadillacs and knew the wiring inside and out, I'd have them take everything apart and install a quality set of comps up front. How much quality can 4" deliver? That's about all the space there is to work with due to the massive armrest/console setup on these Broughams.
Then upgrade all the electricals for having a higher powered sub. Big 3, 200 amp alternator, the special battery.
Once I had that in place I'd be a Real Happy Camper. The car is big. My dream is small...LOL!
Rick