This is a list of unreleased content. Only official content, confirmed in the client or leaked by Riot, is listed. As development of League of Legends is highly iterative and mostly confidential, whether or not an item or feature will be released is highly speculative.
Pending - A piece of content which is being prepared for future release.
Delayed - A piece of content which is being held for technical or other issues, but will probably still be released in the future.
Shelved - A piece of content that has been set aside indefinitely, and may or may not ever be released. Models and/or entries usually remain in client.
Scrapped - A piece of content that has been abandoned due to thematic, copyright, commercial or other issues. Not likely to ever be released. Models and/or entries usually removed from client.
Unknown/Speculation - No reliable information or the only information is indirect, speculative or conjectural.
Was accidentally revealed in the champion selection screen of Voyboy's Mordekaiser rework video.
Has slipped into the game data and swiftly removed. Current status is unknown. No known concept art.
It is currently unknown what this champion was supposed to be. The only known remnants of it are 3 files found in the tournament realm files from the end of Season 6, and Season 3 for Live and PBE. RADS patch 0.0.0.191 for Live, 0.0.1.150 for PBE. 
A champion accompanied by a swarm of insects.
A champion with a more permanent and significant pet.
A champion revolving around interactable units a la Dark Passage.
At first, it manipulated words to create effects. It then became a 'rune mage' that would detonate runes. It was scrapped for being too much of a unique source of power and because of trouble making him look cool.
Riot has bought up loads of domain names surrounding the word 'Supremacy' in relation to 'League of Legends'. While nothing is known about what 'Supremacy' could mean, a similar 'leak' happened for 'Dominion' months before its official unveiling.
Riot will no longer be issuing complimentary refunds on recent purchases that receive significant visual or gameplay changes. Instead, they are now disclosing which champions are set to receive significant changes and are advising players to reconsider their purchases.
New skin splash artwork for all champions that have previously received a visual upgrade. Skins that use the modern splash art standard will not receive an update but may receive some polishing or small changes. Shared splashes will not be updated unless the skin in question receives a new model.
Andrei 'Meddler' van Roon mentioned that Riot was currently getting some work underway to improve the run animation of some of the older champions. They are targeting champs who either aren't in need of a full VGU or aren't likely to get one in the immediate future. Walk/Run anims are being focused on here since they're played more than any other animations.