This isn't meant to be a troll on the Wii, but it probably will be taken as one...
Shenmue as it appeared on the Dreamcast looks better than a lot of Wii games. You could do a direct port of Shenmue 1 and 2 and they wouldn't look out of place. If they improved the framerate and added motion controlled QTE's like RE4 did, then I don't see why they wouldn't port Shenmue 1 and 2 to the Wii. As I said this isn't really a troll on the Wii, it's just that Shenmue looked so good on the Dreamcast, especially in 480p.
They would of course have to put a disclaimer on the box saying:
"Warning, Shenmue II's story ends with a huge cliffhanger, and Sega re-releasing this game on Wii is not a promise to finish the story, or release Shenmue III. If this sells well, then maybe ."
Maybe some sort of standard "Wii Test Game" sticker could be issued to developers from Nintendo, so audiences know not to get their hopes up for a sequel to other games too.
Sega porting more Dreamcast games to Xbox Live Arcade and PSN seems to be a good place for a Shenmue testing ground on those consoles. If re-releases of Shenmue I and II sell well enough, they could green light Shenmue III, even if it is much smaller in scope than most modern adventure games.
Games like Oblivion, Heavy Rain and Mass Effect have raised the bar for what kind of scale we should expect from an adventure game. Maybe Sega feels that to do Shenmue's legend justice, they would have to build something on a similar scale. I say no, no Sega, you don't have to build something that costs millions and millions of dollars. Just release the game anyway you can, on any system.
I still think Shenmue ported to DS would be cool.