The thing is, the Dreamcast is so old and technology has advanced rapidly. Essentially every modern game today would have to be de-made to some extent to even run properly.
If you're talking about modern games being made during and for the Dreamcast era, I'd say Sonic Mania could possibly run. The game is 186MB, which I believe can fit on a CD. Issue is it uses 2GB of ram, so it'd need to be cut down in that department. Maybe even Sonic Origins could run.
Maybe Portal 1? Though I don't know if the portals would be able to run on the system, at the very least I'd expect some frame drops while looking into one.
Maybe even a shooter like Team Fortress Classic could run.