This discussion is great!
Here's a challenge you guys should try: form the Yiddish Empire starting as Levi the Liberator, Count of Lakewood, New Jersey.

As the great Barney Stinson once said.... "Challenge accepted" xD

"The House always wins"
Starting as the ruler of Las Vegas, convert to consumerism, and conquer Flagstaff, Two Sun, Los Angeles and Mariposa.

"The trail of Cheers"
As a ruler of Cherokee culture, have the title of king of Appalachia and control Land-of-Sky.

"NOT AGAIN!"
As a norse ruler, burn the bishopric of Paris (County of Brant, Ontario) to the ground.

"Bee True to your name"
As a mormon ruler of the Antophile dynasty, control the county of Travis, Texas.
(Reference to the town of Bee cave, where the Engineer of TF2 hails from, and the mormon obsession with bees)

"Uranium city here we come!"
As an atomicist ruler, control the county of Edmonton and at least two holy sites of your religion.

"Dethroned"
As a sedevacantist ruler, control St. Louis and have the current Pope imprisoned.

"I O WA"
As the ruler of Iowa, take out a consumerist loan.

"The devil went down to georgia"
As a ruler of Dixie/Tuskegean culture, be posessed"

"Go back to your damn island!"
Starting as a Gaian Canuck ruler, usurp the Haida Tlagaang.

"Ohaidare"
As a ruler of Haida culture, control the kingdom of Ohio

"The ghost dances again"
As a paiute ruler of Death Valley, convert to Nanissáanah.

"Forging the choo-choos of a new America"
As Ishmael Ashur, ruler of Yohogania, control the county of Chattanooga.

"Unsuspicious circumstances"
As the ruler of Gitmo, become president.

"I wish i was in the land of cotton..."
As a dixie ruler, have your army suffer from winter attrition.

"EVERY. MAN."
While controlling the kingdom of Louisiana yourself, grant 5 kingdom-tier titles to vassals

"But yeah, there was a black sheep..."
As the ruler of Kickapoo, discover your son is a member of a satanist cult (whenever those get implemented) and have him banished.

"Who-do voodoo? No do voodoo."
Have your court chaplain convert the last voodoo county on the map.

"Sol Victoria"
As a ruler of Sol Invicta faith, control Two Sun.

"Portland is weird."
As a ruler of Portlander culture, be Occultist.

"Not in name only"
Have at least 20 diplomacy on a ruler, as well as attractive, and reform the Charismatic faith.

"Antibaptist"
Be in holy wars against an Anabaptist and an Evangelical ruler at the same time.

"That's it! Jump in the ocean and swim back to Japan!"
Repel the japanese invasion.

"Kingdom of Dave"
As a jewish ruler, control Washington D.C.

"South Parked"
As the king of Colorado, control a county in south america.

"Remnarnts of our father's legacy"
Beginning as the count of Marquette, Create the kingdom of Superior.

"The Fifty-first"
Starting as the ruler of The Fifty-one, control puerto rico

"The unexpected candidate"
As the ruler of Rushmore, restore the United states.

Heres another challenge :D

"Aztec Rebirth": As a Nahuan character with Mesoamerican religion, create the Nahuan (Aztec) Empire.

[To create the Nahuan Empire, the requirements can be found, by searching the title in the "Find Title or Region" button]

Achievement "good ole days"
- hang a black character

Damn.... The challenges are getting a little dark xD

The "Aztec Rebirth" challenge achieved :D

Start:
http://steamcommunity.com/sharedfiles/filedetails/?id=1344163817

Ending:
https://steamuserimages-a.akamaihd.net/ugc/923675790640520175/AB2F03D4ED0DF17DB0861E024AA77C32EA91C915/

"The other unexpected candidate": Create the empire of Canada and form the confederated faith starting as the duke (or count) of Alberta

"Conquered no more": Control all 5 Americanist holysites as a native with a native religion

"Dixieland": Control the Confederacy, Atlantica and Transallegheny starting from the a southern count

"Bluebonet Highway" : As a Texan or Tejano have a trade post on all traderoute texan provinces
"SIT DOWN" : As an amercanist be at war with a Texan and Californian Empire at the same time.
"SHUT UP" As a Texan or Califonian amercanist kick out one of the invaders.
"We are back" Unite america and have a canadian and mexican (the equivilant) kingdom as a tributary.
"Party like it is 1836" : Unite the Texan empire and with Rio Bravo on your side go to war with every mexican (can not remember all the cultures) realms.

Manhattan Project: As an Atomacist, conquer and convert the County of Manhattan.
Honestly surprised that wasn't on here earlier...

How about this one:

"Born in the USA!": As an americanist create America having it and all its kingdoms (de jure and de facto) as Elective Monarchies.

Here’s an idea ;3;

“Spending Spree”

As a Consumerist, form a de jure or titular with over 50k in the Treasury

FIBS

As a Yooper or Delsman ruler, creat Chicagoland.

True Empire

Restore the Californian Empire with your capital in Imperial.

"Happiest Place On Earth"
Control the County of Orange and the Tribe of the Mouse as a single character.