When I first read this it seemed a bit gimmicky and unneeded, but after thinking about it for a while this as well as items that trigger on heals (similar to how Athene's works) would be a great way of gating items to be more for supports.
I don't think it would be a great idea to have a mass of transforming items, but I could see a 2 item line of these being very good for the game. I'd like to see a more AP stat heavy item which supports like Sona, Lulu and Karma would be drawn to as well as a more utility focused item which Janna, Soraka and Nami would prefer. Since this item is fairly restrictive requiring a heal or shield on your kit to be effective there'd be a fairly low risk of mages picking it up, only a couple could stack it and it would be a slow process.
To start with I'd look at a basic Tear like item
Philosopher's Stone
+
+90g (650 total)
Stats: 20 AP, +25% mana regen
Unique passive: Golden Heart
This item builds stacks of pure gold, up to a cap of 100(?). Each stack grants 0.25% mana regen.
Shielding 10 damage for an ally grants 1 stack of pure gold
Healing 20 HP for an ally grants 1 stack of pure gold
At 100 stacks transforms into a Golden Philosopher's Stone
This item would have 2 upgrades, the first aimed more at AP supports, being a more aggressive tool to help a Sona or Lulu stay relevant in terms of damage
Diplomacy
Golden Philosopher's Stone +
+ 250g (2100 total)
70 AP, 50% mana regen
Unique Passive, Gunboat:
After healing or shielding an ally gain 15% increased ability power for 3 seconds, if your ally is bellow 20% HP when you gain this the benefit is doubled. AP is gained after healing or shielding and will not effect the heal or shield.
Unique Active, Wind Bolt:
Send a shot out in a line dealing 150-200 (based off level) +25% AP damage to the first target hit. This target will take 15% increased damage from your allies and deal 15% reduced damage to you and your allies for 4 seconds. 40 second CD
In addition to this there should also be a more Soraka/Janna style item aimed at more raw supporting rather than trying to have relevant damage
Eleisa's Miracle
Golden Philosopher's Stone +
+
+ 250g (2100 total)
Stats: 25 AP, +75% mana regen, +200 HP +10% CDR +10% shield+heal power
Unique Passive, Aid:
Your healing and shielding spells leave a lingering healing effect, healing both you and your ally for 10+levelx3 HP over 3 seconds, this effect will not stack. While this heal is in effect both you and your ally have +15 MS.
Unique Active, Miracle
Chanel and tether yourself to an ally, granting them a persistent heal for 20% of their missing HP per second. As long as the channel and tether remain unbroken and your ally is alive you can continue healing them. Tether range 700 (visible to both you and linked ally) 120 second CD on end of effect.
I'm not sure about the balance on these items, it's probably abysmal TBH. But the idea was more just to show directions this could potentially go in. The defensive heal/shield space at this point is also incredibly crowded, coming up with something unique was not easy at all.