Matt Horner by SpectralDuke

Level 1:
Supply Lines: Quest: Regeneration Globes gathered by allied heroes near one of Horner's units increase Horner's mana regeneration by 0.1. Reward: After 30 globes are gathered, Horner gains 100 Mana.
Battle-Scarred: Quest: Enemy minions that die near Vikings grant 0.2 attack damage and takedowns grant 0.5 attack damage. Reward: After gaining 40 bonus attack damage, Vikings also attack 20% faster.
Heal Bus: Quest: Heal 5000 damage with Horner's Medivacs. Reward: Increase the energy regeneration of Horner's Medivacs by 200%.

Level 4:
Gatling Good: Vikings deal 100& more damage to Minions, Mercenaries and Monsters.
Sensor Tower: Activate to deploy a Sensor Tower. The Sensor Tower detects enemies within its radius, but does not reveal their identities. Only one Sensor Tower can be active at once. 30 second cooldown.
Scanner Sweep: Activate to reveal an area of the map for 3 seconds and detect enemy heroes within it. 50 second cooldown.

Level 7:
Shockwave Missiles: Vikings gain a secondary attack that fires once every 2 seconds for 50% of their normal damage an deals splash damage.
Evac Flight: Medivacs can now pick up Vikings and transport them.
Got Freedom?: Liberators can now unsiege and move to a new position while being directly controlled.

Level 10:
Yamato Cannon
Close Air Support

Level 13:
Phobos-Class Weapons System: Vikings gain 20% attack range.
Cellular Reactor: Medivacs have an additional 100 maximum mana.
Ultra-Capacitors: Vikings and Liberators attack 15% faster.
Vanadium Plating: All of Horner's units gain 15% health.

Level 16:
You Ready for War?: Horner may now have two squads of Vikings active at once.
Advanced Healing AI: Medivacs also heal an additional target near the primary target at no extra cost.
Stabilizer Medpacks: Units being healed by Medivacs gain 25 Armor.
Advanced Ballistics: The Liberator's Defender Zone is now 40% larger.

Level 20:
Behemoth Reactor: Yamato Cannon gains an additional charge.
The Yamato is Loaded...: The Battlecruisers occasionally fire their Yamato Cannon at enemy Heroes within the target area for high damage.
Starbase: Activate to deploy a Starbase. The base has 1000 health (+4% per level) and 150 mana, and regenerates mana at a rate of 1 per second. The Starbase can use Horner's basic abilities, allowing him to have an additional unit of each type active. Only one Starbase can be active at once.
Point-Defense Drone: Activate to deploy a Point-Defense Drone. The Drone lasts for 6 seconds and grants nearby allies 50 Armor while active. 90 second cooldown.

The idea of Horner was to bring the fantasy of a SCII commander into HotS. This might not have been the best idea, granted, but I wanted to experiment with the concept. As such he has no direct battlefield presence and influences the tide of the battle by deploying units and using calldowns. I chose Horner specifically because he's one of my favourite SC characters and because he's a character who works better as a commander rather than a direct combatant. This also alleviates the issue of Horner being in control of Raynor's ult, since if both are on the team Horner's abilities and ults don't conflict with Raynor (except Yamato Cannon maybe).

Because of Horner's lack of a physical unit, his skins would all be themed abilities and/or voiceover instead, though they could redesign his sprite on the hero select screen too.

Skins:
Fleet Commander

Master Horner:
Themed abilities. All units use mercenary and spec-ops models instead.

Skycaptain Horner:
Matthew Horner is one of the most renowned and trusted skycaptains in the Alliance. Commanding his gunship with great skill and pride, he fights tirelessly to ensure the safety and happiness of the Alliance's civilians. The men under his command swear that there isn't a commander in the Alliance as selfless as he is, though some privately find his idealism chafing.
Themed abilities: Vikings, Medivacs and Liberators are replaced by Warcraft-esque skyships, Yamato Cannon fires a giant cannonball, Close Air Support summons several skyships and fires cannonballs.

General of the Heavens Matthael:
Though his name is unusual, Mattheal's piety and skill are well-known to the Heavens. One of Imperius' foremost generals, he commands flights of angels like they were extensions of his body, slaughtering the twisted forces of the Burning Hells in their multitudes. Privately, he disagrees with his commander's stance on humanity, wanting only to see the lands of Sanctuary prosperous and happy.
Themed abilities: Vikings, Medivacs and Liberators are replaced by angelic units, Yamato Cannon fires a ball of holy fire, Close Air Support summons angelic barges and fires holy spears.

Will update with more talents and possibly some tweaks.

1.1: Completed talent list. Reduced the number of Vikings deployed by Deploy Vikings to 1 to avoid issues with Battle-Scarred.