Quartz
Hero
Lots of stuff already covered so I won't repeat that. I do have one textual issue. Throughout the document you refer to 'd6'. I think you should refer to 'die' or 'dice' throughout the document.
I.e instead of
have
and instead of
have
(change italicised)
This allows for the d6 to be changed to a different die size. For instance, instead of granting the feat at level 10, you might increase the die size from d6 to d8.
All that said, the big one I've spotted is that the only ability that uses the Insight Dice as dice is the Improve Damage ability. So I wonder if you actually need Insight Dice at all? Perhaps instead you have Insight Points and the Improve Damage ability becomes
Improve Damage: When an ally you can see within 30 feet of you inflicts damage to one or more creature(s) you can also see, you can use your reaction to increase the damage dealt by the ally by a number of dice equal to the points in your Insight Pool. Use the dice involved in the attack; d6 otherwise. Afterward, remove one point from your Insight Pool.
(i.e. someone doing damage with a dagger rolls extra d4s; someone using a greataxe rolls extra d12s.)
I also think that Improve Damage is a bit too good in later levels and should have an increased cost:
Improve Damage: When an ally you can see within 30 feet of you inflicts damage to one or more creature(s) you can also see, you can use your reaction to increase the damage dealt by the ally by one die at a cost of one point from your Insight Pool. You may increase the damage by two dice per extra point of your Insight Pool expended to a maximum number of points equal to your Proficiency Bonus. Use the dice involved in the attack; d6 otherwise.
You might also want to restrict Improve Damage to non-magical attacks and attacks that aren't special abilities. For instance, should it stack with Sneak Attack, Assassinate, and Meteor Swarm? How about Cure Wounds used on Undead?
I.e instead of
Afterward, remove 1d6 from your Insight Pool.
have
Afterward, remove one die from your Insight Pool.
and instead of
Improve Precision: When a willing creature you can see within 30 feet of you targets a creature for an attack, you can use your reaction and remove 2d6 from your insight pool to grant that attack advantage.
have
Improve Precision: When a willing creature you can see within 30 feet of you targets a creature for an attack, you can use your reaction and remove two dice from your insight pool to grant that attack advantage.
(change italicised)
This allows for the d6 to be changed to a different die size. For instance, instead of granting the feat at level 10, you might increase the die size from d6 to d8.
All that said, the big one I've spotted is that the only ability that uses the Insight Dice as dice is the Improve Damage ability. So I wonder if you actually need Insight Dice at all? Perhaps instead you have Insight Points and the Improve Damage ability becomes
Improve Damage: When an ally you can see within 30 feet of you inflicts damage to one or more creature(s) you can also see, you can use your reaction to increase the damage dealt by the ally by a number of dice equal to the points in your Insight Pool. Use the dice involved in the attack; d6 otherwise. Afterward, remove one point from your Insight Pool.
(i.e. someone doing damage with a dagger rolls extra d4s; someone using a greataxe rolls extra d12s.)
I also think that Improve Damage is a bit too good in later levels and should have an increased cost:
Improve Damage: When an ally you can see within 30 feet of you inflicts damage to one or more creature(s) you can also see, you can use your reaction to increase the damage dealt by the ally by one die at a cost of one point from your Insight Pool. You may increase the damage by two dice per extra point of your Insight Pool expended to a maximum number of points equal to your Proficiency Bonus. Use the dice involved in the attack; d6 otherwise.
You might also want to restrict Improve Damage to non-magical attacks and attacks that aren't special abilities. For instance, should it stack with Sneak Attack, Assassinate, and Meteor Swarm? How about Cure Wounds used on Undead?