It would be a big thing if they announce Halo 4 and Gears 3 for 360 in 2010. But, if they do that, what are they going to have to move Nextbox? It will be at least as expensive as the 360 was at its launch and Halo and Gears are Microsoft's biggest gaming properties. Microsoft doesn't really have anything else big enough to move a new console other than those games.
And if Halo 4 and Gears 3 come to the 360 burn out will start to set in for a 5th or 4th installment of either series on Nextbox as it does in almost all entertainment series. See how the last three Star Wars films are generally not considered to be in the same league as the first three, how the last Spider-Man film turn ed into a groaner for many fans towards the end, and much the same with Tim Burton's Batman film series and many more.