(Note: I haven't collected al the exotic weapons, and I don't have the DLC, so this is based off mainly reviews and things on this forum.)
Many people have been talking about how week exotics in D2 are, and I sort of agree. A lot of them are pretty weak. Here are some things I'd do to buff exotics.
Rat King: Adding in full auto and increasing the ammo you get while rat pack is active would help this guns main issues.
Sturm: Storm and Stress does practically nothing, so that needs a buff. maybe a PvE impact buff too.
Sweet Business: Astartes Marine recommended that this weapon should have larger reserves and should star spinning while aiming down sights.
Vigilance Wing: The problem with this gun's perks is that you never want them to trigger. How about reviving an enemy will reload the gun, or something else that encourages team play?
Jade Rabbit: There's no point in hitting body shots because you have lower dps, so if you want to do damage, you might a well use something else. Fate of All Fools should increase your dps.
Crimson: It needs it's reserves at least doubled, but I've heard that this is an otherwise great gun.
Coldheart: This needs better damage in PvP.
Fighting Lion: According to Datto's review, this gun is pretty good in mid level activities now that it's been buffed. Maybe getting a kill with it should reload it or increase reload speed so you can kill trash mobs.
Graviton Lance: Here's the problem with this gun: It plays like a scout rifle with crappy stability, and in that sense there is no reason to use this over a scout rifle. Here's what I'm thinking: The first shot deals lower damage but has better stability. The second has normal stability and normal damage. The third has enough damage to make up for the first, along with less stability, no falloff, and better precision damage. That makes it so if you can get past the weird recoil, the weapon rewards you with a faster than average ttk. When you kill enemies they implode, dealing AoE damage and recharging your grenade.
Hard Light: Take away the screen shake! Please Bungie! Increase the recoil if you want, but take away the stupid screen shake already!
Skyburner's Oath: I like this gun how it is, though some think it needs explosive rounds, which I will not object to.
Tractor Cannon: It need's to shoot actual tractors. Please Bungo. Give this to meeeee!
[spoiler] Just kidding! It should probably deal normal shotgun damage, but this weapon was never meant to be crazy competitive. It was meant to be fun. [/spoiler]
DARCI: I've heard that this gun is pretty underrated, but it doesn't really have a good power fantasy, so how about this: Personal Assistant shows the enemies health and precision spot. Target Acquired only triggers while your aiming at their precision spot, but increases precision damage, aim assist, and grants hair trigger. That way it feels like the gun is helping you, and the bonus precision gives will give it great DPS.
Prospector: This weapon will probably be overshadowed by Colony and Telesto no matter what, but for starters it needs sticky grenades. That's probably the best thing it can get. And how about the longer a grenade is out, the higher it's blast radius. That could be great in countdown, or for clearing a ton of mobs.
Legend of Acrius: If shotguns ever become special weapons again, this ones staying in the power slot. Just wanted to throw that out there.
Colony: I think this needs better blast radius on direct hits for PvE purposes, but this weapon is apparently hilarious in PvP, so that's cool.
And that's all the weapons I think need buffs! Comment any feedback or suggestions below.