Hey so- I also was in this camp of "there isn't anything on the dreamcast worth playing that hasn't been ported" for a long long time. Sold my dreamcast. Years went by. Then I started seeing translations pop up and some deep dreamcast cuts that I wanted play. Then the Atomiswave arcades got ported over to the dreamcast, and I was like HECK IT LETS GET ANOTHER ONE. As an aside- I did the same thing with my N64 at the time. I have not reinvested into the n64. Still have a few games, but yikes. Not a lot on that platform.
Picked up a Dreamcast after putting together a short list of games. A lot of the dreamcast's games are kind of- I don't know how to put this exactly.... hot memes? Just wild stuff. I also started to get into not only translated games, but japanese games that have a low language barrier. Dreamcast library opened up in a way I didn't see before.
I haven't gotten a ton of time to play these yet. I'm kind of trying to get through a ps3 backlog first.
Aside from the Atomiswave arcade ports- Here is my list. Mostly deep cuts.
Some games aren't necessarily deep cuts, but I just haven't gotten around to them when I owned a dreamcast.
Confidential Mission
Outtrigger
Cannon Spike
cosmic smash
dynamite cop
illbleed
Ready 2 Rumble Boxing + Round 2
Sword of the Berserk: Gut's Rage
TWINKLE STAR SPRITES
Zombie Revenge
18 wheeler
blue stinger
blue submarine no 6
BOKOMU NO TATSUJIN
D2
Death Crimson OX / 2
elemental gimic gear
FRAME GRIDE
LACK OF LOVE L.O.L.
NAPPLE TALE
record of lodoss war
Segagaga
seven mansions
THE LOST GOLEM
Also- I will say- the dreamcast is *FANTASTIC* modded out. Get yourself a ODE, new fan, new battery and the thing not only works well, but is whisper quiet. That was one of the things that drove me crazy with it. The dreamcast is so loud. Not anymore!
BTW- almost none of these games are "must plays." Instead I kind of consider them gaming oddities. For example? Should you play blue stinger? Sure. Should you play resident evil first because its a better game? Probably.