I believe if Halo 4 is capable of appealing to both casual and competitive players. To do so, it would need a versatile sandbox, varied gametypes, and some form of ranked playlists.
Sandbox: I think all the weapons should be made stronger and harder to use. This would mean that an average player going from Halo 3 to Halo 4 would get similar kill times. This is because although he would no longer be as accurate with the gun, the stronger damage of the gun would allow him to kill in the same time. An exceptional player would be able to get quicker kill times than inferior players, rewarding better aim and skill. One thing the sandbox DOESN’T need is useless weapons. How much would you think before swapping a Plasma Repeater for a DMR? Not much. Why put it in the game if it is completely outclassed by other weapons? Here is an example sandbox I made, where every weapon has its own unique uses(I don’t care if these weapons are in Halo 4, I just want them to be balanced similarly):
I think it would be cool for the Halo 4 AR to have a stun and damage boost with headshots. This would promote using the AR, and help prevent melee rushing. A skilled player using the AR could prevent someone from AR rushing by aiming for the head. It would have other uses in the sandbox as well, such as slowing down objective carriers etc. The AR would take 8 shots for shields, and 6 body shots to kill. A headshot on an unshielded opponent would do double damage, to promote precise aiming. The assault rifle would need to have much higher accuracy (similar to an unscoped BR), lower aim assist and bullet magnetism than it does now. It would also need some sort of recoil system to prevent spraying at the head. As it is now, an AR kill with an unskilled player can take nearly a full clip, and that should be the case with the assault rife. But now, skilled players would actually use the AR. These changes would make the AR useful for kills, as well as giving it some unique advantages in some scenarios.
I think the Halo 4 magnum could work well with a 4 shot kill, high rate of fire, and no scope. The accuracy would be kept comparable to the magnum in Halo 3 to allow it to dominate up close to mid range, but the lack of accuracy would limit long range effectiveness. This would kill faster than the AR, but have less aim assist and lack the stun effect. This is so even though it out does the AR in kill time, the AR would have the benefit of stunning players rushing power weapons, objectives, vehicles, and cover. The AR would also be more accurate, so it would be a better choice at longer ranges.
A Halo 4 BR should have the same accuracy as the Halo 2 BR, be the same difficulty to use in terms of aim assist and mangetism, but the lower rate of fire (relative to the magnum) would keep it from ruling close quarters. At long range, it would have high accuracy, but travel time would call for players to lead.
A Halo 4 DMR could have a 12 shot clip, a three shot kill (2 shots for shields and a headshot/3 body shots for a kill) and aim assist would be slightly higher than the Reach Sniper, and the rate of fire would be same as the Halo 1 Magnum. To prevent players from hitting two body shots with a DMR and switching to an easier weapon for a headshot, the DMR would have a slow put away time. (Switching weapons would take a while) The DMR would be the most effective non power weapon, but only in the right hands.
Their should be no “clones” of human weapons. Each weapon should have its on strengths/weaknesses. They could be similar in role, but not execution. Example, a forerunner long range weapon similar to a scoped sentinal beam. (They have something like it in Reach) It would have the advantage of not needing reload, but have the disadvantage of broadcasting your position. Also you would need to constantly keep your reticule on the enemy to get the minimum kill time.
Power Weapons:
With normal weapons killing much faster than in Halo Reach/Halo 3, the power weapons are now not as “Powerful” as they used to be. However, only the trully skilled players will be able to take advantage of the faster kill times and they can stay mostly unchanged. However, to keep up with the faster game play, they need some buffs.
Rockets: The rocket could actually send spartans flying/knock them down if it didn’t kill them. This could work against you if it sends an enemy crashing into cover, but the splash damage of a second rocket could easily finish him off. Same goes for FRG or foreruner clone.
Sniper Rifle: Keep it similar to the one in reach without anti-vehicle properties. The Halo Reach sniper is easier to use than halo 3’s, but it would stand a better chance against the new BR etc.
Sword: Now the sword is ineffective against assault rifles, since an AR can keep you pinned with the stun effect. To help it out, change it fundamentally a little. First off, sharply reduce the lock on effect the sword has. This means you have to aim at the person you are about slice up, not look in his general direction. Though this will make the sword harder to use, we will add a new primary attack. This primary attack will make you dive in the direction you aim. (Similar to evade) If you hit an enemy with the attack, you kill him, and recover from the dive. Otherwise, you have to recover while vulnarable. You could use this similar to evade, but it would be balanced by recovery time.