excuse me for getting stuff wrong if i do cuz its 4 in the morning and i cant sleep and been looking around the forum reading stuff, but it seams like halo has always have a common weapon that rules all and if you are good with head shot accuracy for a quick kill(not including super weapons).
halo ce: pistol or die
halo 2/3: BR or die
halo wars: giant army or die (jk not including this game)
halo Reach: DMR or die/Anni: pistol or die
halo 4 should have more of a weapon balance(using only a few weapons not all), my idea’s are:
more speed in these Spartans and no sprint!
able to break away from assassination with knife
pistol-
slight damage and range decrease, really fast reload and makes less noise for stealth
assuming a Battle Rifle in halo4-
slight damage reduction, no bloom, best for long/med range
Assault Rifle-
accuracy increase,more range for med/near range, damage increase
less effective for far ranges
SMG-
less ammo, faster fire-rate, damage to take down shield until reload, near range
Sniper-
slightly more delay before firing a second shot, no range/damage change,
should slightly push the cross-hairs up like real weapons
shotgun-
should hold less ammo (about 4-5) pumping action takes slightly longer, close range
Rocket Launcher-
1 rocket max per reload, delayed fire, 3 instead of 4 ammo,
faster bullet speed, slightly less AOE
Spartan Laser-
takes 2 shots to take down vehicles except small like mongoose, harder to aim
pummeling- (thinking should be knifing)
takes 3 hits to kill and only damages health
DMR- (what? we want some weapon variety :P)
causes slightly less damage, has bloom, slightly farther range then BR
grenade-
plasma= same dmg as frag (insta kill on stick)
frag= same dmg as halo 3
Grenade launcher-
mode 1: fires a mine like grenade, sits and exploding if enemy near
mode 2: fires a grenade that explodes on contact
plasma pistol-
better lock on, emp’s for only 1 sec
energy sword-
2 hits to kill, first bounces off shielded armor (no shields anymore) blown away from shield dispersal, second stabs and kills. slight speed increase but no charge.
Needler-
lock on takes more accuracy, slightly farther range
beam rifle-
takes more time to cool down and pushes up per shot
gravity hammer-
instant kill on contact, pushes user and enemy back if not, causes some damage
plasma rifle-
increased accuracy, slight damage improvement
these are only a few that I came up with, hope u likey 
this would make weapons more usful in different situations, ex: BR beats AR at far range, AR bears BR med/near range, super weapons will be fore quick decisive kills when they arise on 2 or 3 enemies before switching to primaries