First off, GTA on the Wii wouldn't sell nearly as well as all us Nintendo fans would hope it would. It certainly wouldn't sway a lot of mature content onto the machine. We have seen this happen with Eternal Darkness and RE4 already.
The only way it would happen is if Nintendo can gaurantee a million units sold in about a week, and that's a high mark for them to reach given their historic userbase. This might be possible when there's several million in the US, but that will take until March at the earliest. This only is possible, though, if Nintendo has managed to drag in a good number of new gamers and the casual set that still wants to play mature games. Unfortunately, there's no way to tell if that group has successfully grabbed the Wii.
In other words, unless there's a healthy population of "hardcore" (ugh I hate that word) owners and enough new-to-Nintendo-but-solid-gamers owners and casual gamers who only play GTA and the other bigger franchises, there's no way Rockstar is going to bite.
Unless Nintendo drops a huge money hat on their head, which is possible, but not normal Nintendo MO, so I don't see it happening. They've always used a "are you good enough to be on our system" approach, and hardly the other way around. Granted this has lessened in recent years (hello Iwata), but it's still a pretty high attitude.
As for GTA itself, I don't care for the series, but if it appeared on the Wii in a workable form - not some bullshit spin-off - I'd pick it up at launch on the principle alone that I do want to see this kind of content on the Wii. If it means paving the way for more developers to bring me a wider variety of games, then I'll do it for [almost] that reason [alone].
As for the comparisons between, say, GTA Wii versus GTA 360, there's no adequate way for us to tell. Obviously GTA 4 will have some sort of upgraded engine (or it damn well better, as I still say it looks like shit and always has...which is ironic now that I think about it - cuz that means the gameplay is the main thing, which would be in line with Nintendo's strategy for the most part) that will use a lot of buzzwords and technologies that gamers will positively respond to, which would preclude that specific version from happening on the Wii.
Instead, the Wii would have to get a scaled down (graphically, I mean) version of GTA 4. It would have to have Wii-centric controls, and while it seems perfect for this, there's no gaurantee Rockstar wouldn't just add in aiming and call it a day. I don't think Rockstar is lazy, I think they would be cautious, and they know their audience lies primarily on the other systems.
I.e., unless the Wii controls were so super awesome that it gave way to massive sales, Rockstar is going to shy away.
Which means that if currently historical-but-brief evidence shows, I could see Rockstar giving us a slightly upgraded Wii-enabled version of Vice City, or perhaps a remix of all the PSP games, or an amalgamation of the Xbox/PS2/PSP games. This would almost certainly happen before anything original was placed on the Wii, because Rockstar would want to test the waters prior to commiting to a full game.
At the end of the day, I don't think it will happen. Nintendo can't gaurantee the sales. If a version does appear, count on it being a port from the older systems. If it sells well, then we might get an exclusive version.