The Rune that needs to be in Precision.
TLDR: Mages and Fighters cannot properly use the Precision tree because their are not enough good options. By adding a rune that gives CDR (Legend: CDR - up to 10% max) fighters and more importantly Mages would have more reason to use this tree.
Precision is supposed to be the tree that is designed for DPS. To me this tree should be used primarily by 3 classes. Marksmen, Fighters, and Mages. This is only partly true now because the tree keeps getting changes that are for marksmen or are a result of marksmen. Presently, Marksmen dominate this tree, while Fighters only opt into it for Conquerer, and the only mage that used this tree effectively was Kassadin, until they reworked Presence of Mind.
Fighters primarily use Triumph because Overheal is too weak in a solo lane, and Presence of Mind is only really effective if you have mana, which half of the fighters don't. They also use Legend: Tenacity because Legend: Lifeline is garbage and not all fighters can make use of Legend: Alacrity. Most fighters can use Coup de Grace and Last Stand, but Cut Down doesn't do anything for them. So for fighters, they can use about 4/12 minor runes.
Mages use Triumph and Presence of Mind because again Overheal is just too weak on sustain mages. They can't use the Legend: Alacrity (unless you're maybe Ziggs and running a split push strat with a mage), Legend: Tenacity, or Legend: Lifeline. They can use Coup de Grace and maaaaybe Cut Down, but Last Stand is mostly useless unless you are a battle mage. This leaves them with about 4/12 usable minor runes.
For both classes this isn't a lot. Especially 2 classes that have so many DPS champions that should want to use this tree. Fighters were thrown a bone when Conqueror was introduced, and with Fleet Footwork's AP ratio getting buffed on the PBE, it could finally be usable on mages, so keystones are not much of an issue.
Because of this I am proposing a new Rune to the Legend set of runes. Legend: CDR (because I can't think of a better name).
The numbers could be something like: Gain +2% (+0.8% per Legend stack) CDR, up to 10% with 10 stacks.
This would give DPS mages a usable Legend rune that they can use if they wanted to use this tree as a primary (If they wanted to run Fleet Footwork), or secondary (if they wanted Presence of Mind, Cut Down, etc.). This would also give Fighters another option to use. The Lifesteal is garage, the attack sped is not always usable, and Tenacity is not always necissary, so having CDR would be a breat alternative to some fighters like Riven, Darius, Irelia, Renekton, etc. This can also be used on ADC's, specifically those use Essence Reaver like Xayah, Lucian, Sivir, Ashe, etc. because the item is losing 10% CDR overall and this rune can make up for that loss.
