NPCs

I personally think that all of the pre-existing Dota 2 characters should be at minimum level 6 (the level in-game where you should have all of your abilities unlocked).

Source 1, Source 2

Strength

 * Raigor Stonehoof, the Earthshaker
 * Davion, the Dragon Knight
 * Kaolin, the Earth Spirit
 * Beatrix Snapfire, the Snapfire
 * Elder Titan
 * Huskar, the Sacred Warrior
 * Phoenix
 * Purist Thunderwrath, the Omniknight
 * Sven, the Rogue Knight
 * Rizzrack, the Timbersaw
 * Razzil Darkbrew, the Alchemist
 * Io, the Wisp
 * Kunkka, the Admiral
 * Tresdin, the Legion Commander
 * Mangix, the Brewmaster
 * Rigwarl, the Bristleback
 * Rattletrap, the Clockwerk
 * Bradwarden, the Centaur Warrunner
 * Tiny, the Stone Giant
 * Rooftrellen, the Treant Protector
 * Ymir, the Tusk
 * Mars, the First Son of Heaven
 * Karroch, the Beastmaster

Intelligence

 * Mireska Sunbreeze, the Dark Willow
 * Rylai, the Crystal Maiden

Strength

 * Abaddon, the Lord of Avernus
 * Chaos Knight
 * Crixalis, the Sand King
 * Slardar, the Slithereen Guard
 * Vrogros, the Underlord
 * Undying, the Almighty Dirge
 * Ostarion, the Wraith King
 * Barathrum, the Spirit Breaker
 * Banehallow, the Lycan
 * Balanar, the Night Stalker
 * Leviathan, the Tidehunter
 * Pudge, the Butcher
 * Magnus, the Magnoceros
 * N'aix, the Lifestealer
 * Mogul Khan, the Axe
 * Lucifer, the Doom