Recent evidence suggest that having Konami, Tecmo or Capcom won't make a big difference. Metal Gear Solid woudn't be a megaton level event, in my opinion - Metal Geal Solid 3 release was surprisingly mute. Same goes for Resident Evil 4.
I'm not sure about the impact of SquareEnix - it could be gigantic in Japan and result indirectly in better support, but in the West and outside the group of hardcore gamers, it doesn't matter so much.
Namco would be quite big - having three extremely strong franchises - Ridge Racer, Tekken and Soul Calibur.
The biggest spectacles receently have been Halo 2, Half-Life 2, GTA San Andreas, Need for Speed Underground 2 and Burnout 3. Out of those franchises, Microsoft has Halo exclusive and shouldn't have trouble getting Valve games. EA games won't be exclusive any way, which leaves us with GTA San Andreas.
Getting next-gen GTA exclusive, in my opinion, would allow Microsoft truly dominate in the West, both Europe and America. It would then have all of the top-selling franchises, many of them exclusive.
This, in turn, would force the Japanese publishers and developers take Xbox 360 seriously, as the struggling Japan market won't support them. A Japanese producer friend of mine said "Japan - it is very bad market situation, very depressed. Very bad."
I'm sticking with my 2004 prediction that Microsoft will rock this generation - I'd would go as far as predicting a 35-40% marketshare.