I'd say both Monkey Ball and Beach Spikers were destined for Dreamcast to begin with. I hope one day we get some sort of confirmation of this. Virtua Fighter 4 I reckon could have been on DC you know. I obviously have nothing to back on apart from an opinion but I reckon Sony paid Sega for it exclusively to be on PS2, looking at beat-em-ups on the Dreamcast it seemed quite capable.
Again Virtua Fighter 4 only came out on the PS2 long after the Dreamcast had been discontinued. Had Sega had a console then they probably would have made it exclusive, but they had lost to the PS2 in terms of hardware sales & that was the console that you'd make a fighter for. While Super Monkey Ball ended up on the Gamecube because Nintendo were aiming the console at that type of market.
You know Virtua Fighter 4 launched on PS2 in March 2002, yes? Sega were still developing games for the Dreamcast in the west until mid-2002 and in Japan until far later so the game didn't although came out a year after Sega discontinued the system, they were still supporting the machine. It's no secret that Yu Suzuki was planning to release the game on DC if they could get a perfect port - he didn't want a compromised version - given time and the right optimization I reckon the machine could have had a decent port, it's not unrealistic to think Sony paid for the rights for the game as they did publish quite a few of the early Sega stuff for PS2, it could have been a sweetener to have the bulk lot and Virtua Fighter exclusive.
Judging from the E3 build and just some good old guesswork, I think there's a pretty good possibility that a Dreamcast version of Monkey Ball was in the works at some point. When Sega went under and stopped producing the Dreamcast, many games originally planned for Dreamcast releases ended up on other platforms. In this case, we got Super Monkey Ball for Gamecube. And to those who say the Dreamcast couldn't have handled it, that's nonsense. Dreamcast was a competitive set of hardware, I'm certain it could run Monkey Ball no problem. Maybe there's an old Dreamcast devkit out there somewhere that has that E3 build on it, but who knows...
^But there is no E3 build!? Sega didn't show it off at E3 in spring of 2001 - instead, only showing off a Gamecube version at Tokyo Game Show ~6 months later in fall of 2001. And then the game was released for Gamecube in fall of 2001 and that was the end of things in terms of previews of Monkey Ball/Super Monkey Ball 1...
The list on http://en.wikipedia.org/wiki/List_of_Dreamcast_games doesn't show that Sega were developing many games themselves by then and the few they did weren't of the same pedigree as Virtua Fighter 4 & probably wouldn't have been worth porting. They published plenty of 3rd party games, but I think you had to pay them to do that.