Good thread. I was actually wondering this same thing. I've been predicting that after the Halo buzz, MS could start fading away without another huge title. I definitely know a lot of people who bought a 360 pretty much just for Halo.
But yeah the only big 360 titles that i can think of for '08 would be Halo Wars, Splinter Cell, Too Human, Endwar, PGR, and Alan Wake. Out of those I feel like the only really big title seems to be Alan Wake and perhaps Too Human, although I generally don't hold out much hope for games that have been in development forever (example: Perfect Dark Zero). There could be a new Dead Rising, but considering its a Capcom game, it'll probably be multiplatform.
Can any 360 gamers chime in? It is still pretty early to be speculating about '08, though.
My Top 5:
Shadow of the Colossus, Metal Gear Solid 3, Shenmue, Skies of Arcadia, Chrono Trigger
My 2 nex-gen systems: PS3 
and Wii 
Prediction Aug '08: We see the PSP2 released fall '09. Graphically, it's basically the same as the current system. UMD drive ditched and replaced by 4-8gb on board flash memory. Other upgrades: 2nd analog nub, touchscreen, blutooth, motion sensor. Design: Flip-style or slider. Size: Think Iphone. Cost: $199. Will be profitable on day 1.
