If there's a Halo 3 then it'll entirely depend on the hardware available at the time, the system limitations of the hardware (for example the game may be so graphically advanced that the PS2 can't handle it - KOTOR anyone?) and the publisher and developer involved deciding on whether there's a large enough market for the game on the console.
For example if Halo 3 is announced and due for release around the same time that the Xbox 2 is released, the Developer/Publisher may decide that the Xbox 2 doesn't have enough sales to warrant releasing the game on that format.
Also on the point quoted, Halo 3 won't be a "predecessor", it'll be a continuation of an already existing and VERY strong Brand. It's a similar situation to the Championship Manager Series, Hitman Series, Tomb Raider Series, Battlefield Series etc etc.
BTW guess what industry I work in... lol
