It would be cool if Nintendo makes a firmware upgrade that allows Wii owners to play Turbo CD games and Sega CD games.
Why would they do that when they could just build it that way in the first place?
It would make sense if they did it that way. In the earliest announcements about NEC/Hudson being involved they stated that CD games would be supported, but NOTHING has been said about it one way or the other since. Due to the space issue, I would think some sort of build in emulation wherein you could just insert an actual SegaCD/Turbo CD game instead of downloading it would be ideal.
That would be the best root in general, and what Nintendo has historically gone with. There's no emulation issues because the hardware is THERE, like the Gameboy Player, or the GB/GBC processor in the GBA and the GBA processor in the DS. Even Sony did it this way, with the PS2's sound card basically doubling as a PS1, the rest of the hardware effectively being "off" when a PS1 game is playing. It would also be, to me, the ultimate reward for fans of the companies that actually have a bunch of Turbo CD laying around. You can be the coolest kid on the block again by being the only person who can run "that whys game with the red haired guy" in its full CD audio glory.