I'm a long-time DreamCast owner here, having picked mine up not long after the initial NA release in 1999. While my DC remains unmodified, I did pick up the keyboard adapter, mouse, VGA, and LAN module at some point in the distant past, and even booted NetBSD on the console on a few occasions (mostly at local LUG meetings!).
After sitting unused on a shelf for some years (along with a second DC a friend gave us several years back!), I've decided to start tinkering with the DreamCast once again, as well as introducing my 6 yo to the platform (his first game on it was last night, playing Sonic Adventure. As predicted, he loved it!).
My original DC now has a dead CMOS battery, which I'm looking to change out (miraculously, the second unit I have still holds a charge, though I'd imagine it probably doesn't last long once it's unplugged!). I'm also considering a USB or SD mod to protect the discs I have (we're trying to teach the 6- and 3-yos good optical disc etiquette, but it takes time!). I've been falling down a rabbit hole ever since looking at things I could do to modernize this system, eventually landing here.