In all honesty when i first found that they were giving medallions and "adding" their power to links i thought maybe link would gain some bonus abilities, such as water medallion acting as a surrogate scale so you wouldn't need the gold scale to dive deep sadly it wasn't the case, did learn later that it was meant to be a thing just never got added. In terms of what they could've done i always envisioned it as:
Saria: Call to green things to create ivy to climb up surfaces or to
immobilize a target temporarily, or to summon a deku baba to spit a seed
Darunia: To give link a bit of extra weight and strength (Used in combo with megaton hammer) Or to act as a stronger din's fire
Ruto: Honestly i always thought Nayru's Love would've been perfect for Ruto's power, she's the only one who seems to love Link, or to allow link to dive deeper than before.
Impa: To let link hide in plain sight for a short time and to have a larger bow capacity - could you imagine Gerudo Valley with this?
Nabooru: Give link a greater chance of drops that you actually need instead of just hearts all the time.
Rauru: This one i felt was taken by navi truthfully, the light medallion could've given link hints on what he should be doing and ways to solve a puzzle, maybe highlight where links gotta go with a glowing light.
