I know Dreamcast has some of the best fighting games, but they've just never been my thing. I grabbed Aerowings, Airforce Delta, Virtua Tennis, and Shenmue on ebay. All for pretty cheap. I'll grab Skies of Arcadia and Grandia 2 once I'm done with Lunar (on PS1) or if I find a good deal.
Man, this is like just in my wheelhouse. I loved me some Dreamcast...I hope you got all 4 controllers with it, because i feel like this system was designed specifically with couch co-op in mind.
But you are on a pretty good path already...
I loveddd Skies of Arcadia, and Gradia 2. I hope both of those games hold up over time...
Powerstone 2 was really good as a couch-co op game, and Toy Commander was also a really fun couch co-op game.
I want to say MDK2 was a dreamcast game..although the graphics on that game might be getting old.
San Francisco Rush 2049 was a fun, and really ridiculous racing game. I remember the weird moon physics, and doing tricks with my cards and stuff...was pretty cool with friends also.
Tony Hawk Pro Skater 2...Such a good game....just so damned good--but it was certainly available for other platforms.
Chu Chu Rocket, and Samba de Amigo were great--although you need to buy weird maracas peripherals for Samba de Amigo, but that shit was fun...
Marvel vs. capcom 2...great if you love fighters...Soul Calibur was also great...
I loved Crazy Taxi as well, and i know everyone who played Jet Grind Radio, or Jet Set Radio really loved it, but i never did buy it for whatever reason...
Either way, great trip down memory lane, I am sure there are other titles, but i cannot remember them off hand.
Edit: Someone mentioned Hydro Thunder...that game was really fun also. Oh, and Ready 2 Rumble was not a bad boxing game, a little goofy, but fun. Also, i know it is kind of sacrilegious, but i think Shenmue was a little bit overrated. I have also heard people mentioning Trickstyle...i would not get that over all the other racing games people have mentioned...it was kind of an unwieldy racer...