Oh, and what's the new threat gonna be? You killed the prophet and destroyed the covenant, you destroyed gravemind and pretty much destroyed the flood(i think). The only thing i can think of would be all of spark's robot friends since they turn against you at the end of Halo 3 and an entire game of sentinels and robots would be so boring. And remnants of the covenant would also be boring.
in case you dont know, spark is the little ball thing that's super hard to kill.
And people still ignore the fact that halo 3 was supposed to be the final showdown thing and now they are making halo 4.
The game play may be fun but i think the campaign will suck monkey tit balls