Some suggestions for face of the mountain, stoneborne fact, greenfathers gift, and bandit.

It is a pain in the ass getting stacks efficiently with this item lategame. 80% of the time people(yes, I'm talking to you mid laners) will just burst down the minions and not let supports use the stack. Your teammate is basically denying you gold, and you literally have to "fight" for the farm. If you compare this item to or they require no communication with your team whatsoever.

A suggestion to improve this item is to consume a stack automatically if a nearby minion is killed by a teammate when you have AT LEAST 4 stacks(maximum), or when a cannon minion is killed. Another suggestion is to increase the maximum number of stacks over time, because you reach 4 stacks fairly quickly. This way you can use the stack to actually heal your teammates in a teamfight. The current state of this item late is just a grinding tool, where you have use up all the stacks on a minion wave because you never know when you will be able to use the stacks.

Stoneborn pact is the replacement of the old keystone mastery "bond of stone". It will heal allies for 5 + 2.5% of your maximum health over 2 seconds if they autoattack a target that has been cc-ed by you. This mastery is under average early game, decent mid game, but weak late in my opinion. The reason I think this is under average early is because it is the only keystone that require timing for it to be usefull. The support have to apply the cc, and the adc have to autoattack that target to consume the buff, which basically means it is only usefull in "all-in" or skirmishes, which rarely happens on the supports it is designed on(braum). All in supports(leona) usully go colossus.

Tanky support usually start with 700 ish hp, and get around 1k hp after the first or second recall. With 1k hp the mastery will heal for 30 over 2 seconds, or 15hp per second. In mid game you will usually have 2k hp(if you build properly), and it will heal you for around 30 per second(which is quite decent), but later in game you will invest your money on armor and magic resist, so you won't really get much over 2k hp. When your adc gets bursted from 100 to 0 in 2-3 seconds it doesn't help with 40 regen per second. Windspeaker doesn't fall off lategame and gives 22 armor to the ally, which is supergood lategame. 22 armor on a 2000hp adc will make him around 400hp more tanky if it makes it easier to understand. I think a slight increase in scaling for this mastery lategame will make it more appealing and better.

Greenfathers gift is a mastery that makes you deal 3% of the enemies CURRENT hp as bonus damage(9 sec cooldown), if you step in a bush(which limits this mastery to supports only, and only for the ranged ones). The problem I have with this mastery is it deals damage based on current hp. This mastery is made for the aggressive poke supports who can utilize the bush in laning phase. This mastery is basically useless in mid game and lategame. In laning phase enemy ADCs usually have 600-1000hp. This mastery will deal 30 damage(even less if you take account of magic resistance) if the enemy have 1000hp, and less if you have poked down the enemy. This mastery should do damaged based on max hp, not current.

Bandit - This mastery gives 3 gold for every autoattack you do on enemy champions (10gold for melee). I am fine with the way it works for ranged champions, but for melee champions it is nearly impossible to autoattack the enemy unless you really commit. A reasonable buff would be to make melee champions proc this mastery by hitting enemies with spells(5gold per spell).