I think with a little forethought given to it a truly AWESOME Advent Calendar could be made. The concept is simple. Take a character/unit from the fluff/old codicies that no longer exists. Commission a short story for this character/unit. Have the studio make a model and a dataslate. Make all of your December releases these models/units. This would be a great way to get models into the hands of eager fans and over come the "No model, no rules" trend.
So here is a proposed list of what to do this for. NOTE there is no Warhammer Fantasy, only 40k BUT I think it proves the point that this is a valid idea since I could list 25 separate items they could do this for. I am sure there is plenty in the fantasy universe to do this with as well, so maybe they could do it 2 years in a row.
This list is in no particular order. All links are to their particular Lexicanum article.
1- Moriar the Chosen.
Former Captain of the Blood Angels, this Death Company Dreadnought has been a long favourite of veteran BA players, even though he has been gone since the 3rd edition supplemental codex. The coolest engine of destrruction to walk the field of 40k IMO, I know the next DC dread I build WILL be Moriar.
2- Grand Master Mordrak. Missing from the current iteration of the Grey Knights Codex, this Grand Master and his Ghost Knights are a victim of the current "No Model, No Rules" trend. A greatly lamented loss to the codex and an opportunity for some awesome fluff on Huron Blackheart vs GK.
3- Duke Traevelliath Sliscus. Another victim of not having a model. This paragon of the Dark Eldar race (read MONSTER) and adversary of Lukas the Trickster would be easy to bring back into the fold.
4- Veteran Sergeant Cluetin. Found in the old Angels of Death Codex, this Blood Angel is the keeper of the Shroud of Sanguinius, the remnants of a banner that covered the primarch's corpse on Horus' battle barge, the Vengeful Spirit. It is supposed to drive the BA into an even greater frenzy when it's stasis field is deactivated, so maybe a rage inducing bubble?
5- Baron Sathonyx.
Another no rule dark eldar, the Lord Hellion would present an AWESOME opportunity for a particularly cool and cruel model, and would be a addition (or re-addition) rejoiced by DE players everywhere.
The models are even still available from GW. A fan favourite, made famous by Dan Abnett's novel series, these 4th edition guardsmen would be welcomed by many as a formation/dataslate with a character and some sort of detachment, especially if redone in plastic.
The greatest son of the Imperium, based on the exploits of Alexander the Great, and recently featured in Williams King's Macharian Crusade trilogy, it would be grand to see him on the table. A tactician that would make Creed blush, he has plenty of awesome opportunity for cool rules and stories surrounding his conquests. Plus, he already had a model once!
Another group that still have models available from GW. Another 4th edition legacy that could have an awesome character (even more bad a$$ than Straken IMO) and some sort of cool squad/formation.
9- Deathwatch Kill Teams. Something I am surprised doesn't already exist (last time I saw them was either the 3rd edition chapter approved version or possibly a 4th ed update to that). GW even sell a conversion set for them. They are definitely a favourite of Black Library, their current short story submission window even revolves entirely around short stories involving these Xenos hunters. A great opportunity for a formation to pair with Codex Inquisition and other forces of the Imperium, especially with the recent Tyranid resurgence. And if rules for a DW Librarian are included (something that should be in reference to their old rules and current fluff), it is a great way to get in some psychic defense without taking an HQ slot.
10- Lady Aurelia Malys. Yet ANOTHER No model Dark Eldar character who disappeared with the newest edition of Dark Eldar. This rival of Vect would provide another strong female character to the game and some good psychic defense if her immunity is kept in-game.
.This Slanneshi Daemon Prince would be sooooo much fun to have around again (and please alot of 3rd Ed CSM fans). Sure Kor'sarro Khan may have captured his head, but his body escaped. A real cool story revolving around their reunion would be an excellent way to reintroduce this character into the modern 40k universe.
12- Erebus or Kor Phaeron. While they have never been in the game, they really should be. These are the architects of the fall of Lorgar and Horus, and are still at large in the 41st millenium. With Dark Apostles being included in the codex, at least one of these dark lords is a must have in the game.
13- Orkimedes. This Mek boy is the brains behind Thraka's weapons of war and is currently one of the greatest threats to the Imperium. In the Armageddon codex and 4th ed orks, he would be great to bring back now that Stompas exist in game.
14- A Warp Spider Phoenix Lord. One of the coolest aspect warriors, and yet the only one without a named Phoenix Lord. Really something that could be explored in the fluff and would be fun to see on the table, and has such great potential for an awesome model.
15- Sergeant Lukas Bastonne. A Cadian Veteran Sergeant, this 5th edition upgrade character disappeared with the advent of the Astra Militarum (probably to stay a true Imperial Guardsmen) due to the fact he too lacked a model. A hero to all of Cadia, it would be nice to see a Cadian equivalent to the Catachan Gunnery Sergeant Harker back in the game.
16- Anval Thawn. A Grey Knight Justicar who went out since there was no model to represent him. This guy carved his was out of Ku'Gath and is the Imperium's answer to Lucius the Eternal with his immortality and ability to resurrect. An upgrade character that I for one think should return to life in game as well.
17- Asdrubael Vect and the Dias of Destruction.
The greatest Dark Eldar character and the greatest victim of the No Model, No Rules policy. While he may still come out in some sort of supplement, he and his custom Raider are just begging to be redone is some over the top amazing fashion, and have tons of potential for a cool story.
18- Harlequin Solitaire. One of the most fearsome combatants in the galaxy, Harlequin Solitaires would make great Lord Of War Characters for both DE and Eldar (I remember using the unofficial codex once and charging like 24" getting 11 power weapons attacks at something like I7). They represent the most feared individuals of that enigmatic race, and could add tons of fluff into the game.
19- Chaplain Xavier.
A legacy of 3rd ed SM, he is the greatest chaplain the Salamanders ever had, even though he is dead he would still be fun to play with (Tycho is dead, and Blood Angels still have him, even though he is terrible now). It would be nice to see Salamanders with something other than He'stan, and he could lead to such a cool resculpt, and I am sure plenty of conversions based on that sculpt.
20- Gabriel Angelos. The only other guy on this list that never actually had rules, still included him because of his importance to GW. Chapter Master of the Blood Ravens and star of the Dawn of War games, he is an obvious choice to incorporate into the game. Include a chapter tactics for Blood Ravens and you are onto something.
21- Drastanta. Phoenix Lord of the Shining Spears, yet never represented in game. Currently "dead" but a resurrection story and reclamation of the celestial lance would be a cool story and add a cool character to the Eldar range.
22- Anghkor Prok.
Probably the worst name rip off GW ever did (Angkor Wat), this Kroot Shaper would be a critical xenos addition to this list and a way to possibly make Kroot more desirable on the table.
23- Brother Bethor, Chapter Ancient of the Dark Angels. Another Angels of Death Codex Character, this keeper of the Dark Angels three sacred standards. And he is even a recent character fluff wise, having risen to his position only as recently as 996.M41.
24- Sapphon, Dark Angels Grand Master of Chaplains. The last of the Angels of Death characters that are not in the game presently, Sapphon is not as great or old of an interrogator as Asmodai, but he has found himself at the head of their order due to his charisma and battle prowess.
This 3rd Ed SM Crimson Fist Character is possibly one of the toughest Space Marines to ever exist. Currently missing in the lore, his good friend Kantor has hope he is still alive and will return. Such a story and an appearance in the game would bring closure to the matter and be ultimately fun I believe.
GW could even take this concept in other directions, putting out supplemental dataslates with new Space Marine chapter tactics and their associated special characters. But that is a post for another time.
So what do you think of the overall list? Anyone I missed (I am sure there definitely is, but this is all I could recall). Such an Advent Event would win over tons of fans I imagine and could fill an entire month with awesome releases.