Now you can look over every good, bad, and possibly neutral DLC ideas in just one place. Zombies your thing? Demand moar brainz! Zulu invasion? Post it here, debate-- rage, and spam your greatest and worst DLC ideas, and perhaps someday hope to see your grand vision bask in the eternal glory of a $2.99 chunklet of Crusader Kings 2 awesome sauce on Steam or Gamersgate.
Zulus invent biplanes 800 years ahead of schedule and use the speed and ease of travel, and the gold and diamonds mined from the hills of their native land (South Africa), to fly over the perilous Central African jungles and searing Sahara Desert to invade North Africa and Europe. Zulus get a massive bonus to troop speed, can have stone-dropping biplanes that don't have much of an offensive effect, but literally cannot be touched (-100,000% damage from enemies); they can also see 2 (maybe 3?) provinces/sea zones ahead instead of just one.
Dynamic seasons allow for the Saami people of Lappland (northern Scandinavia) to invade and take over most of Scandinavia each season, cyclically. Upon winter's end they get negative bonuses (so, maluses) in all combat-related modifiers and traits; all rulers in the North Germanic culture group get a special casus belli "Reclaiming the Homeland". If the Saami aren't destroyed in time (about 100 years or so), their maluses disappear to simulate them getting more desperate to run away...from what you ask?
They will eventually take over Scandinavia and start ravaging Denmark and Germany, and the Thing will appear in the farthest north of the map. Landing in the Arctic, this alien life form has a hive mind; though thousands of individual bodies are controlled, they work as one, with one brain, and of one organism. It can also shapeshift...it already has spies in every court in Europe. With a unique religion and culture, the Thing just made the game 100 times harder.
Only beaten by finding and inviting an Irishman named MacReady to your court. You must give him your highest title, plus actual demesne land, to allow him to destroy the alien menace.
An Atheist DLC in which susceptible characters (cynical, honest, diligent, quick, genius, ambitious, etc.) renounce all forms of religion and use science and logic to persuade their lieges and friends. +1,000% to tech growth, -200 relations with religious heads, -100 relations to bishops, imams, etc., -50 relations with (zealous, arbitrary, slothful, slow, inbred, content, etc.), -10 relations with all religious characters. They will either be crushed quickly or slowly take over Europe, heralding a golden age of peace and prosperity.
An Army of Darkness DLC and event chain where the player character discovers the Necronomicon Exmortis. Scholars assure that it is safe, and can be used to contact other worlds and dimensions, but a random stutterer in the character's court utters a few choice words in just the wrong way. Corpses dig their way up through the earth as skeleton armies are raised by Undead kings. The condition spreads if not contained, with every province gaining hundreds of "Undead Rebels". Within a year of defeating them, more rise, equal to the amount of living men that died fighting the Undead a year past.
Terror only ends with the player's character defeating and beheading his Undead predecessor in battle (if his predecessor is still alive [usurped title, etc.], then it is the most recently deceased ruler with that title. If the title was newly formed, then it is the most recently deceased holder of a random title of the player character's).
A wild accident in the Byzantine Emperor's court involving harlots and onion pie results in giant mutant cockroaches that threaten to tear the very fabric of space-time itself.
Men such as Leonardo da Vinci and Albert Einstein find themselves in 11th century Constantinople, where they use their knowledge of radiation and physics to turn the bad bugs into goo. But beware! Time travelers such as Adolf Hitler (fresh from a bunker in 1945) and Caligula pool their resources to develop evil Nazi superweapons for the cockroaches to utilize; they would like nothing better than to destroy the Universe!
Can you, as Emperor of Byzantium, solve your bug problem?