Here's how I would have ended the game.
We still have the final cutscene, but instead of cutting to credits, Master Chief jumps to Earth (or whatever) and lands in the middle of a big battle in a destroyed city. Waves after waves of the various enemies comes after you and the few standing soldiers. In the sky, you have civilian spaceships trying (without success) to escape the ennemy fire. We get the usual widescreen bars level introducing a new level with the name "Home Coming Party" or something
Then the whole place starts shaking with big, *boom* sound effects. Then, at the horizon, you see a squad of giant mechwarriors-type robots coming toward the city, using energy weapons to blow up whole blocks. Then, in a cutscene, coming out of nowhere, one of them teleports from a portal and lands right next to you. The Halo theme starts. A group of soldiers try to attach (or slowdown) the mechwarrior by using hooks shot from Scorpion tanks, without any success. The tanks get thrown around into building and soldiers. This is to show the incredible strength of the mechs.
Then the control comes back to the player. A soldier gives MC a gun outfitted with one of those special hooks with a comment like "Have a try,MC". The player has to run to the roof of one of the few remaining building. In a crazy action sequence, MC hooks himself up to the Mechwarrior and climbs inside the machine.
The last level consist in MC climbing up inside the robot, shooting random ennemies, until he gets to the control room, where he managed to hijack the control of the mech. Then the player starts destroying the other mech warriors. All this of course, to the sound of one of those epic sounding audio track. It sounds really dramatic and heroic.
Cutscene:
In another galaxy, we see one of the Prophet in a big white room saying "Enough of this, purify the whole area. The demon must be stopped". An alien voice is heard saying "But Prophet, many of our brothers are still fighting in the area" only to be cut by the Prophet saying something like "They will die for the cause, like martyrs"
So we get the typical 'death star loading up to shoot something'-type sequence. A big cannon mounted on a covenant ship fires this very large white laser ray toward Earth.
We then see the Master Chief sliding down a rope from his mechwarrior, with Marines celebrating the victory. As one of the sergent is saying "Earth has found a new hero, but the job is not ove-" he gets suddenly get cut by statics filling up their helmet radios "my god what is th--".
The whole screen goes white. We hear soldiers screaming thru their radios. then pure white noise for a few seconds.
The colors fades back in. There's not a single thing standing, only big piles of random building crumbles. Everything is covered in white dust; it looks like it snowing, but its just the white dust fallouts. The only thing we hear is the sound of ruins burning down and a strong wind
The camera pans slowly toward a pile of burning metal (aka the remains of the hijacked mech). As it zooms into it, we notice the the helmet and left arm of a very inconsious MC barely making it out of a pile of debris. Of course, heavily damaged. MC's visor is complely smashed in. The Green paint has pretty much melted off, because of the intense heat from the laser beam. As you start to expect MC to start moving, the camera cuts, credits.
Finishing the game at legendary gets you to hear the echo of MC's health warning going on and off in a very odd manner, as the camera zooms toward him.
The whole "is our hero dead?" angle always works well. Well it works better than ending it before the moneyshot