Sadly there's a limit to what you can get from the mystery skin box.
Skins that are "NOT" eligible for the Mystery Gift is:
Pax Champions:
Pax Twisted Fate
Pax Jax
Pax Shivar
Championship Skins:
Championship Ryze
Championship Riven
Miscellaneous Skins:
Judgement Kayle
UFO Corki
King Rammus
Urf the Manayee Skin
Riot Skins:
Riot Girl Tristan
Riot Squad Singe
Riot K-9 Nasus
Riot Graves
Riot Blitzcrank
Victorious Skins:
Victorus Jarven
Victorious Janna
Victorious Elise
Pre-order Skins:
Black Alistar
Human Ryze
Collector's Edition:
Goth Annie
Silver Kayle
Huntress Shivar
Refer A Friend Skins:
Grey Warwick
Medieval Twitch
The reason why...
Listed skins were unlock by achievements and special requirements not pertaining to purchase through the store.
Holiday Skins are unknown concerning this release, but are unlikely to be apart of the Mystery gift events (Excluding Harrowing Skins)
TPA Skins are also may or not be included in this event:
TPA Nunu
TPA Mundo
TPA Shen
TPA Ezreal
TPA Orianna
Skins that "Are" Eligible:
Justicar Aatrox 975
Dynasty Ahri 975
Midnight Ahri 750
Foxfire Ahri 975
Stinger Akali 520
Nurse Akali 975
Blood Moon Akali 975
Silverfang Akali 975
Golden Alistar 390
Longhorn Alistar 520
Infernal Alistar 975
Emumu 520
Almost-Prom King Amumu 520
Little Knight Amumu 750
Sad Robot Amumu 1350
Bird of Prey Anivia 520
Noxus Hunter Anivia 520
Hextech Anivia 750
Blackfrost Anivia 1820
Prom Queen Annie 520
Frostfire Annie 975
Reverse Annie 975
Freljord Ashe 520
Sherwood Forest Ashe 520
Woad Ashe 520
Queen Ashe 975
Amethyst Ashe 975
Boom Boom Blitzcrank 520
Piltover Customs Blitzcrank 1350
iBlitzcrank 1350
Apocalyptic Brand 750
Vandal Brand 520
Cryocore Brand 975
Resistance Caitlyn 520
Sheriff Caitlyn 520
Officer Caitlyn 975
Arctic Warfare Caitlyn 750
Desperada Cassiopeia 520
Siren Cassiopeia 520
Mythic Cassiopeia 975
Gentleman Cho'Gath 1820
Loch Ness Cho'Gath 520
Jurassic Cho'Gath 975
Battlecast Prime Cho'Gath
Hot Rod Corki 520
Urfrider Corki 975
Lord Darius 975
Bioforge Darius 975
Woad King Darius 975
Dark Valkyrie Diana 975
Corporate Mundo 1820
Executioner Mundo 520
Rageborn Mundo 975
Soul Reaver Draven 1350
Gladiator Draven 975
Death Blossom Elise 975
Shadow Evelynn 520
Tango Evelynn 520
Frosted Ezreal 520
Explorer Ezreal 750
Pulsefire Ezreal 3250
Spectral Fiddlesticks 520
Bandito Fiddlesticks 520
Fiddle Me Timbers 750
Surprise Party Fiddlesticks 975
Nightraven Fiora 750
Royal Guard Fiora 520
Atlantean Fizz 520
Tundra Fizz 750
Fisherman Fizz 975
Enchanted Galio 520
Commando Galio 520
Gatekeeper Galio 1820
Spooky Gangplank 520
Minuteman Gangplank 520
Special Forces Gangplank 975
Sanguine Garen 520
Commando Garen 520
Dreadknight Garen 975
Rugged Garen 750
Steel Legion Garen 1350
Hillbilly Gragas 520
Gragas, Esq 975
Vandal Gragas 975
Oktoberfest Gragas 975
Jailbreak Graves 975
Mafia Graves 975
Pool Party Graves 1350
Blood Knight Hecarim 975
Reaper Hecarim 975
Arcade Hecarim 1350
Blast Zone Heimerdinger 520
Piltover Customs Heimerdinger 975
Aviator Irelia 520
Nightblade Irelia 520
Infiltrator Irelia 975
Frostblade Irelia 1350
Tempest Janna 520
Hextech Janna 975
Frost Queen Janna 975
Forecast Janna 1820
Commando Jarvan IV 520
Dragon Slayer Jarvan IV 750
Darkforge Jarvan IV 975
Vandal Jax 520
Jaximus 975
Temple Jax 750
Nemesis Jax 975
Full Metal Jayce 1350
Debonair Jayce 975
Mafia Jinx 975
Sakura Karma 520
Sun Goddess Karma 750
Statue of Karthus 520
Grim Reaper Karthus 975
Pentakill Karthus 750
Deep One Kassadin 520
Pre-Void Kassadin 520
Harbinger Kassadin 975
Mercenary Katarina 520
Bilgewater Katarina 975
High Command Katarina 750
Sandstorm Katarina 975
Viridian Kayle 520
Battleborn Kayle 975
Aether Wing Kayle 1820
Deadly Kennen 520
Karate Kennen 520
Kennen M.D. 975
Arctic Ops Kennen 975 2
Mecha Kha'Zix 1350
Monarch Kog'Maw 520
Lion Dance Kog'Maw 1350
Deep Sea Kog'Maw 975
Jurassic Kog'Maw 975
Prestigious LeBlanc 520
Wicked LeBlanc 750
Acolyte Lee Sin 750
Traditional Lee Sin 520
Muay Thai Lee Sin 975
Pool Party Lee Sin 975
Defender Leona 750
Valkyrie Leona 520
Iron Solari Leona 975
Pool Party Leona 975
Bloodstone Lissandra 975
Hired Gun Lucian 975
Bittersweet Lulu 975
Wicked Lulu 975
Dragon Trainer Lulu 1350
Sorceress Lux 520
Spellthief Lux 750
Commando Lux 520
Imperial Lux 975
Steel Legion Lux 975
Coral Reef Malphite 750
Marble Malphite 520
Obsidian Malphite 975
Glacial Malphite 1350
Shadow Prince Malzahar 520
Djinn Malzahar 520
Overlord Malzahar 975
Charred Maokai 975
Totemic Maokai 520
Assassin Master Yi 390
Chosen Master Yi 520
Ionia Master Yi 975
Samurai Yi 520
Headhunter Master Yi 975
Cowgirl Miss Fortune 750
Waterloo Miss Fortune 520
Secret Agent Miss Fortune 975
Road Warrior Miss Fortune 520
Mafia Miss Fortune 975
Infernal Mordekaiser 520
Pentakill Mordekaiser 975
Lord Mordekaiser 975
Exiled Morgana 975
Sinful Succulence Morgana 975
Blade Mistress Morgana 750
Blackthorn Morgana 1350
Ghost Bride Morgana 975
Artic Warefare Caitlyn
Retired Skins that are listed in Mystery Gift:
Skins retired on the 28th of November, 2011
Pharoh Amumu
Red Riding Annie
Toxic Dr. Mundo
Mr. Mundoverse
Nottingham Ezreal
Masquerade Evelynn
Scuba Gragas
Swamp Master Kennen
Shamrock Malphite
Sonoran Kog'Maw
Dragon Knight Mordekaiser
Leopard Nidalee
Sasquatch Nunu
Blacksmith Poppy
Professor Ryze
Yellow Jacket Shen
Hextech Sion
Spectacular Sivir
Badger Teemo
Highland Tryndamere
Kingpin Twitch
Skins retired on the 12th of March, 2012 [2]
Nightmare Cho'Gath
Unmasked Kayle
Emerald Taric
Butcher Urgot
Skins retired on the 16th of March 2012[3]
Crimson Akali
Matador Alistar
Safari Caitlyn
Desert Trooper Garen
Angler Jax
Phantom Karthus
Vizier Malzahar
Noxus Poppy
Workshop Shaco
Frozen Shen
Feral Warwick
Time Machine Zilean
Skins retired on the 17th of July, 2012 [4]
Annie in Wonderland
Red Baron Corki
Alien Invader Heimerdinger
Firefighter Tristana
Magnificent Twisted Fate
Skins retired on the 19th of July, 2013 [5]
Hextech Galio
Sailor Gangplank
Caterpillar Kog'Maw
Molten Rammus
Muse Sona
A bit more then that, sorry I don't have the full list
Reference/ Credit :
Tough Guy - http://forums.na.leagueoflegends.com/board/showthread.php?t=3993112
