I feel like you could use his treacherous nature to really interesting effect in a MOBA - like, maybe a random chance for Sargeras to show up mid-game and mess with the character, converting him to fight on the enemy side. I have no idea how you would even begin to go about balancing that or making the player invest in one team's victory when they might defect at any moment but it would be an interesting challenge.
As it is Medivh in HotS is all portals and ravens and ranged mystical shenanigans. Here's a basic rundown:
Instead of mounting, Medivh can transform into a raven, increasing Movement Speed by 15%. While transformed, Medivh can see and fly over all terrain and is immune to all effects.
Q - Arcane Rift
Launch a rift that deals damage to enemies in its path. If an enemy Hero is hit, reduce its cooldown by 5 seconds and refund 30 Mana.
W - Force of Will
Protect an allied Hero from all damage for 1.5 seconds.
E - Portal
Create a set of portals between you and the target location, allowing allies to teleport between both. The portals last 6 seconds.
R - Poly Bomb
Polymorph an enemy Hero, suppressing their attacks and Silencing them for 2 seconds. On expiration, Poly Bomb spreads to other nearby enemy Heroes.
R - Ley Line Seal
After 0.5 seconds, unleash a wave of energy that imprisons enemy Heroes in Stasis for 3 seconds.
What I'm getting from this, first and foremost, is that you can create an infinite Mexican wave of people turning into sheep with one of your ultimates if the enemy team doesn't scatter. The other thing that sticks out is that this is more about moving around and helping allies/hindering foes, so it's more about the "standing on the cliffs, using magic from afar" flavour of Medivh.
Here's a video of the character in an actual match via streamer KendricSwissh - it's on the Haunted Mines map and the player confirms that there are bugs on that particular map when it comes to Medivh so maybe bear in mind this isn't quite how he's supposed to work, particularly when the untargettable raven form is being targetted...
I believe he's due to land this week, which is fortuitous for me because I've been getting back into Heroes of the Storm recently - just for kickabouts and odd matches rather than competitive play - and I play ranged mages. My other friends seem to have largely fallen off with it though, either being lured away by Rainbow Six Siege and then on to other games, or simply preferring the Overwatch button on Battle.net. Are any of you still playing?