First of all, it has been confirmed the game's name has something to do with "Destiny", so that is what I'll refer to as I go on.
What I'd like to see in Destiny is, first off, a complex plot. Halo: Combat Evolved was set in the future with little to no information on the plotline was given (unless you read the manual, like I did) and you woke up onboard a ship being attacked by an alien force as you approached a mysterious artifact, a ring-world. As you proceeded, you found hints of something far more dangerous than the Covenent aboard Halo. Then, about two-thirds of the way in, you met the Flood, and all hell broke loose. No one saw the Flood coming. No one expected the sheer terror of being attacked by what was formerly your own men, and as you watched helplessly as your men, even your captain, turned into mindless killing machines hall-bent to consume the galaxy. It was a suprise that no one forgot. Destiny should have a plot that leads you into an indepth world where so much is alien, yet so much is familiar. And then, just when you think you have everything figured out, the world flips upside down. Perhaps it shouldn't be done exactly as "343 Guilty Spark", but similar in the fact that you never see it coming. Not a sudden betrayal, not a new enemy, but a sudden revelation that makes you question your actions up to this point.
Secondly, I would like for Campaign and Multiplayer (Perhaps even Cooperative Play) to be joined. Reach had a fair idea with customizing your Spartan, but I want to go more indepth. First of all, they should the modes together, like Borderlands had it. The gameworld can be explored solo, but you could come across other players. To prevent too many people being in one spot, the gameworld should be vast, at least several dozen miles in any direction, going over rough terrain to smooth deserts to abandoned jungle temples. There should be a "hub" city or some form of area where players can meet and socialize, maybe make teams for exploring the vast world, or challenging them to player versus player matches to improve both of your combat abilities. Furthermore, there should be "outposts" further out in the wilderness that are like small bases to prepare you for whatever lies outside. There could be small cities that lie abandoned, with little inhabitants nearby that require your assistance. You could visit with your team and together stop the evil that threatens the small border town, and be rewarded with information on your ultimate objective. It should feel like the world is living around you, and everything reacts to your actions and the actions of your teammates. Even if you didn't accidentially shoot the boy's pet rabbit, you should still get repercussions since you were in charge of the party.
Thirdly, customization. Every one wants their character to be unique. Now, their appearance should, first off, play into their abilities. A certain body armor should give better coverage towards one type of projectile to another. For instance, Kevlar takes the heat from a bullet, but when Plasma gets involved, it doesn't stand a chance in hell. On the other hand, an energy shield can absorb the damage of a plasma gun, but bullets pass right through as they are solid and plasma energy. That way, there are checks and balances. Secondly, weapons should be customizable. Rather than give several weapons with no real customizaiton other than one has a scope and another doesn't, there should be a good variety of weapons where you can play around with. A good, slow firing weapon with good damage can be upgraded to be a long-range DMR with a enhanced zoom scope, infra-red target spotter, and perhaps even a built-in microphone to listen into conversations so those stealth ops go easier. Silencers, bayonets, and so on allows for the player to have versatility. A good team could have a marksman sitting back and keeping an eye on the team while another uses explosives to draw attention away from the squad member assigned to steal secret documents from inside the compound. There should be no perks, however, as the uniform and player abilities according to what class they choose (Should that be the system) will do enough.
Finally, the game should have an overreaching feel of mystery, exploration, and something hidden deep within the world(s?) of Destiny. Everything you find should play in later, and not be cannon fodder for the markets. Sell something that you thought was a useless trinket early on, and you'll suffer later. There should be a bank system, naturally, to prevent hoarders from being overloaded. Furthermore, the game should reward players who explore. Hidden weapon upgrades, paths around hostiles, et cetera will cause those who search the most to win the most.
Oh, and one more thing. With or without achievements, the milestones that one reaches should play into more than money and missions. The higher you go up the levels, the more you know and act. New paths that you wouldn't have seen as a green recruit are now obvious because you've been in combat situations where your squad leader spotted something you didn't. If possible, perhaps the stimulus that causes the ranking up should play into what is advanced on your character. For example, if one ranks up during a stealth mission then the character should be better at stealth, perhaps even learning abilities such as how to kill silently, impersonate guards or enemies, use cloaking devices more efficiently, et cetera.
So, opinions and suggestions?