Okay I got two ideas that I like and would personally like to do, had I an empty Dreamcast shell.
1) Make it into an Arcade Stick. Not quite sure how structurally sound it is, and how much space you have, but it would be awesome.
I was hoping to find a pic of someone who had done similar, but this is the closest I have
You could even make it so the Start button was the power button etc.
2) Buy a second hand Saturn/MegaDrive/Master System etc and put the electronics from another Console inside the Dreamcast shell. That way, you could have a Sega Saturn disguised as a Dreamcast, or a Mega Drive disguised as a dreamcast. With the Mega-Drive you could make it so you need to open the disc tray to put in the cartridge! A sega Saturn would probably work the same way. Not sure how hard that would be, but I imagine it could work well, and if you managed to pull the DC guts out, you could probably swing it.