The Sega Dreamcast is one of my favorite consoles of all time. It was too beautiful for this world and it died far too young. :/
There are plenty of great Mods for your Dreamcast. All dreamcasts should have their battery swapped with a Battery holder / new rechargeable battery and a self healing fuse to prevent the controller port from bricking. Replacing the Gd-Rom with a GDEMU clone or a MODE by Terra Onion is ideal for the modern age and for the convenience of having the entire library stored on the console.
I have a Video on the GDEMU here
If you're more interested in having being able to play discs from any Region / being able to play burned discs on the later VA2 console, then a bios mod is what you're after.
I have a bios mod video on a VA1 console here!
If you have a VA0 or VA2, check out the written guide here
There are plenty of other great mods for the Dreamcast that I've yet to cover Such as the DCHDMI Mod and Internal HDD mod for Dreamshell.
Incase you didn't know, the dreamcast is natively capable of playing burned discs. If you have a stock Dreamcast laying around, grab some CDIs and burn them with IMAGE BURN. Here's a great video by MrMario 2011 showing you da weigh