I reckon the next game should be Halo: Return of the Forerunner
After the war, 10 years to be exact, everyone has settled down nicely. A covenant ship is found coming out of slipspace, so the humans prepare their defences in case the ship is hostile. However just before the order to give fire, the master chief radios in "This is Spartan-117, do not attack, repeat do not attack." A lieutenant general replies back "Master Chief? Where is Cortana?" Chief replies "She is currently in the hands of her makers, who are also on this ship" The Lieutenant replies "What? The Forerunner?". Once the ship lands, the forerunner meet the humans and they litterally look exactly the same, exception of clothing. 10 mins later, a covenant armada, consisting of all the Holy cities makes a final attack on earth. The covenant foolishly mistake the forerunner for "human" vermin and attack them as well. As the forerunner detonate a slipspace explosion, as seen in halo reach, they inadvertantly create a new organism. At first, the humans and forerunner alike study this new organism, but soon it turns disasterous and it attacks everyone in the room. The parasitic growth then, after learning its abilities, transforms up into a grey table. The forerunner who survived called it a "shape shifter", meaning its able to turn into anything it wants. Before it attacked, the forerunner were able to determine that the shape shifter was liable to the effect of the halo ring. Having known this, they head out to the nearest installation, being 04. However, when they arrive and the forerunner find their ring destroyed, they curse humanity for possibly dooming the existence of the entire multi-verse. Realising that bickering was pointless, chief recommends heading toward the ark, being only a mere 145.58 lightyears away (for a slipspace that is). The ark had fully repaired itself and the new installation 04 was currently waiting idle for the correct permissions for it to be moved to its new location. Once the forerunner land at the arks control room, they order everyone human but the chief to head straight back to earth on one of the forerunner ships, collect everyone from earth and travel via slipspace to the leo madra galaxy on the other side of the universe. Once they exit the jump, disaster strikes. Shape Shifters had assumed the shape of the forerunner ships, therefore rendering it highly risky to actually activate the rings. The chief turns to the forerunner command officer "The universe has been nice to me. Time to return the favour. Activate the rings. There are at least 3/4 of forerunner still with my race. Do it, its time to return the favour." After the chief resigns to his fate, he notices only 1 forerunner ship acting out of fashion. Example, its exterior was glossier than anything else. 'Time to make my final kill' he thought to himself. He calmly waltzed in front of the fake ship and said aloud in an evil tone, a tone that meant he doesn't care if he died, just that so long as he knew he did his part for humanity "Come out you disgusting parasite. Show me who's better? Me? Or just something that can transform into anything it wants?!"
The chief should have watched his mouth. As soon as those words left his mouth, his mind felt like it was being stabbed with a dagger. 'They can read our thoughts and minds' he thought. He immediately saw a picture of 343 GS in his mind. He heard a mocking laugh which then said aloud "ahh, the perfect weapon".
Instantly, the ship dissapeared and thousands of small, floating heads appeared. Chief, armed with forerunner weapons, which consist of a Forerunner Reaper, shoots the 343 GS beam far more concentrated, and a Forerunner modified Spartan Laser, which shoots the same beam, but this time it runs on oxygen, therefore rendering infinite ammo. You would now enter a firefight mode where you have infinite waves of 343 GS attacking you until you as the chief eventually die. When you die, the forerunner activate the rings. Every single being in the Orion arm wiped out in a matter of minutes. The humans return to earth. A newly assembled spartan team had journeyed to the ark to reclaim the masterchiefs helmet. After the Eulogy, the game slowly ends by fading towards the masterchiefs destroyed helmet, which is just a shard from the back of his helment saying "UNSC Spartan-117"
[Edited on 03.21.2011 2:04 PM PDT]