Look to Battletech to balance Fusion Rifles (Destiny)
So I posted this on reddit, and I'm not sure it even got looked at... maybe it could generate some discussion here.
For those of you who don't know this is battle tech
A weapon called the PPC (particle projection cannon) is a high damage all or nothing shot, with long recycle times with high heat. A unique aspect of this weapon in battletech is that it has a minimum range in addition to its maximum range. The lore goes that the weapon has safeguards in place that inhibits the field that holds the charges particles together to keep close discharges from damaging the mech it is equipped on.
Given the formerly one hit kill close range overlap of the Fusion Rifle and Shotguns, I've been thinking of a way to bring the FR to its former glory, and give it a place within Destiny. Give it a minimum distance that starts to do damage within the damage falloff of the longer range shotguns. This combined with removal of the damage and some percentage of the spread nerfs could give the FR a deserved niche within the game.
In terms of getting the mechanic to fit in with the game lore is that my understanding is FR's are essentially are a weaponized fusion reaction, current science has been using magnetic fields for fusion containment. Why couldn't the FR spew out plasma created from the fusion reaction of the weapon and then only switch on the magnetic containment fields to shape and give a vector to the shot? Not unlike Cortana's upgrade of the Covenant's capitol ship plasma weaponry in the book Halo:First Strike, the addition of tighter magnetic fields gave it much more destructive power compared to the their plasma mortars. Given the relative strength of ourselves and our enemies the unguided plasma is just a tickle.
The bigger question is, are Destiny's weapon damage models capable of doing this? Ultimately, would doing this have another far more drastic consequence in terms of the weapon meta?

Look to Battletech to balance Fusion Rifles
It's certainly a neat idea. At close range I don't think it would hurt too much since you're already going to get beat by a shotgun most times. It becomes harder at longer ranges but it might work. See the Queenbreaker's Bow for a fusion rifle that is actually a sniper rifle! You're right though, we don't have any indication that I'm aware of, that Destiny can decrease damage output as a target gets too close. I think every weapon at this point simply becomes more powerful or stays at the same level of power the closer it is to an enemy.
Also, good job relating your idea to Cortana's modification of the Covenant plasma weapons. I'll take you a step further back and relate what Durandal, one of the Marathon's three original AIs, was able to do with a captured Pfhor vessel in Marathon II:
We have the answer, but I'm afraid that
like the ancient S'pht I may be destroyed
before being able to use it. I must bring
you up to Boomer.The Pfhor clearly intend to capture my ship
intact, and I can't blame them for wanting
to reverse-engineer all the improvements
I've made to their stolen technology. I'd
have erased my seven times table to hear
what the combat technicians on the Khfiva
shouted when they learned I could focus a
particle beam at nearly twice their maximum
range.Today I have forced the Pfhor Naval Academy
to update its curriculum. The Third Battle
for Beta Tear must be dropped from the
Seven Great Battles which every aspiring
Pfhor naval officer must memorize and
replaced with The Humbling of Battle Group
Seven at Lh'owon.
:)
I hate fusion rifles. remove them all from the game, solved!
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How about one of these?

God damn I love Durandal.
One of the finest characters of all video game time.

And that was when I realized Bungie had a thing for 7.
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