Hypothetically, would the Wii be able to have such games, albeit with downgraded graphics of course, without drastically changing the game?

I ask because I read somewhere that COD 5 will be essentially the same game on the Wii as is on the HD consoles. I am thinking how is that going to work.

If in "drastically changing the game" you are not including graphics, then yes, it's possible.

Yes, but it would just have bad graphics in comparison. None of these games seem to have any physics that would be too hard for the wii to do. Gta4 does have some good Ai though, but the wii might be capable of this.

The controls would have to be different also to fit the wii mote and nunchuck...
unless they want to use the gamecube control.

yes it could have em like them but with weaker graphics

well...gta=no.
im not sure how well bioshock would transfer over...technically possible, but i dont know if it could keep the feel.
mgs....my understanding is that its pretty linear and nothing overly special (gameplay). size is the real issue. but i guess it could be placed on a number of discs.

Yes, of course. These types of games were on the PS2. I would rather them not be on the Wii, however.

Not to mention the game won't be in HD, so you wouldn't have to worry about size.

im not fully aware of the specs of mgs but even without the nicer graphics would it not still be rather large?

I doubt Bioshock would transfer over well either because there was some pretty complex AI involved with the Big Daddies.  I'm not too tech savvy so maybe I'm wrong, but how is the Wii at handling complex AI?