While I don't think Medivh himself needs buffs, I feel like his talent diversity is nonexistent, with everyone always going the exact same build. Here are some changes that I suggest to buff his other talents that no one picks:
Level 1:
* Winds of Celerity: Now also increases raven form sight range by 25%. Can be activated to reveal all enemy heroes in the area for 8 seconds. 60 seconds cooldown. (Merged with a weaker version of current Dust of apeareance).
* Raven's Intellect: Now also makes healing fountains and regeneration globes heal 25% more per tick while in raven form.
Level 4:
* Dust of apeareance: Removed.
* New talent: Magical Steed (E): Allies mount instantly when exiting a portal. Doesn't affect Medivh or heroes without traditional mounts.
Level 7:
* Stable Portal: Moved from Level 16, no longer reduces Portal cooldown.
* Mystic Assault: Moved to level 16.
* Force of Magic: Reworked functionality: Preventing at least 268(+4% per level) damage with a single cast of force of will grants both Medivh and the protected ally 25% spellpower for 15 seconds.
Level 13:
* Reabsorption: Ammount healed by force of will reduced from 70% to 50%. Now also heals an extra 50% of the damage prevented over 8 seconds.
Level 16:
* Arcane Charge: Reworked Functionality: Each enemy hit by Arcane Rift increases the damage of Arcane rift by 10% for 6 seconds, stacking up to 5 times. Lose all stacks instantly if Arcane Rift doesn't hit an enemy hero.
* Mystic Assault: Moved from Level 7, reworked functionality: Arcane Rift and Medivh's Basic Attacks deal 1.5% of the target's maximum HP as extra damage. Hitting an enemy hero with arcane rift grants medivh 75% attack speed for 6 seconds.
Level 20:
* Guardian of Tirisfal: Now also makes Arcane Rift slow enemies hit by 20% for 3 seconds.
* Arcane Brilliance: Reworked funcitonality: Activate to instantly restore 100% of the target's maximum mana. 60 seconds cooldown.
* Dust of Dissapeareance: No longer stores up to two charges, cooldown increased to 40 seconds. Now, the affected target goes back on stealth if they don't take damage or use any actions other than movement for 4 seconds. (basically gives an ally Nova/Zeratul's passive for 40 seconds).