We should not convince you to get a Dreamcast. This decision should be made by you for your own reasons. I do not mind recommending some games for you though. I will say this, it sounds like you have already played the original XBOX.
If you like RPG"s I would go with the already mentioned Sky's of Arcadia and Shenmue. Shenmue 2 was released for the XBOX in the united states. The Dreamcast did get Shenmue 2 in JPN and PAL though. Shenmue is more than an RPG. it has elements from just about every genre. From fighting, Racing, Adventure, Acton, Role Playing, Arcade and Simulation. For the arcade potion you can walk into a arcade in the game and play some classic SEGA games such as Hang On and Space Harrier. For the simulation aspect they make you get a job and you actually have to perform your work duties which consist of driving a fork truck moving creates from area to area.
Now for a few that you may not have known about. Grandia 2, I cannot recommend this game enough it is one of the best RPG's that I have ever played. The combat system makes it a lot more enjoyable than turn based or hack n slash RPG's. Game Arts has re-released it for windows pc, the Grandia® II Anniversary Edition can be downloaded on steam and GOG.com.
Next we have Phantasy Star online v2, PSO still has an active online user base. Dreamcast-talk hosts a server "Vega" where DC-Talk members meet up for up to 4 player online coop game play. PSO is a hack n slash style RPG and different people have different reasons for playing it. SEGA has made many sequels to the game ranging from version for handhelds such as the Sony PSP and Nintendo DS to the newest one Phantasy Star Online 2 on the PS4 in Japan region only.
Next we have Record of Lodoss War. This one is a bit hard to describe. It is a overhead action RPG Like Diablo 2. Evolution 1 and 2 are both pretty good. Nice game play and story. Evolution 2 did not get a English voice overs like the first one though so you will have to read a lot. There are about 20 more, but these are the few that I enjoyed. A couple of honorable mentions are Time stalkers and Armada. Well that about does it. I linked to a few videos so that you can see some game play and watch a few reviews. Good luck with choosing, I personally have a PS2, XBOX and Dreamcast. I use to own a Gamecube too but I lent it to a friend and never got it back.
Phantasy Star Online V2https://www.youtube.com/watch?v=WYx8QyzLWX4
Record Of Lodoss Warhttps://www.youtube.com/watch?v=OOJHep3art4
Evolution: The World of Sacred Device https://www.youtube.com/watch?v=cAz6UgMxass