Easter Eggs

[Incredible Bulk]

Many other mods including vanilla. It has settlements and people name after book, TV and movie characters. Also artifacts from an outside source or real life.
In vanilla you have relics.
Certain unique historical followers like Peter the hermit or Marco Polo
The city Arguin in the very SW corner of Africa
Timbuktu and Dongola's Ivory, Slaves, and Gold mines (not really an easter egg for veteran players, but it sure feels like you've hit pay dirt when you A) find these as a new player and B) send a halfway decent merchant there
Getting any of the difficult to get Guild HQ's such as Mason HQ (THE hardest HQ to get by far), Horse Breeders HQ (as a Euro faction), or Swordsmith HQ (as an Islamic faction)
Realizing a few scripted events are actually based fairly heavily on history (such as the Albigensian crusade on Toulouse, game equivalent is when France is excommunicated, the Pope calls crusades on Toulouse more than any other city)--I'm not talking about the obvious ones like Black Plague or Gunpowder, but the ones that are scripted to happen more often than others.
Getting to know all the little nuances of the game (faction relations, reputation, vassals, military assistance, chivalry, dread, piety, shoot arrows into non shielded side, crusades/jihads, how to use pikes effectively, how to use muskets effectively, etc) and how they really work; it really makes M2TW a completely different game to play.

The Thera mod There are many 'easter eggs' contained in the mod, names of famous people from history, movies and literature, along with settlement names with links to mythology, sci fi, history. There are even characters named after personalities from the TWC.

What about TATW are there Easter eggs? Reference to characters or places outside of the middle-earth world? It those of Middle-earth but not everyone knows they are there? Hard to find?

[Feanaro Curufinwe]

Many other mods including vanilla. It has settlements and people name after book, TV and movie characters. Also artifacts from an outside source or real life.
In vanilla you have relics.
Certain unique historical followers like Peter the hermit or Marco Polo
The city Arguin in the very SW corner of Africa
Timbuktu and Dongola's Ivory, Slaves, and Gold mines (not really an easter egg for veteran players, but it sure feels like you've hit pay dirt when you A) find these as a new player and B) send a halfway decent merchant there
Getting any of the difficult to get Guild HQ's such as Mason HQ (THE hardest HQ to get by far), Horse Breeders HQ (as a Euro faction), or Swordsmith HQ (as an Islamic faction)
Realizing a few scripted events are actually based fairly heavily on history (such as the Albigensian crusade on Toulouse, game equivalent is when France is excommunicated, the Pope calls crusades on Toulouse more than any other city)--I'm not talking about the obvious ones like Black Plague or Gunpowder, but the ones that are scripted to happen more often than others.
Getting to know all the little nuances of the game (faction relations, reputation, vassals, military assistance, chivalry, dread, piety, shoot arrows into non shielded side, crusades/jihads, how to use pikes effectively, how to use muskets effectively, etc) and how they really work; it really makes M2TW a completely different game to play.

The Thera mod There are many 'easter eggs' contained in the mod, names of famous people from history, movies and literature, along with settlement names with links to mythology, sci fi, history. There are even characters named after personalities from the TWC.

What about TATW are there Easter eggs? Reference to characters or places outside of the middle-earth world? It those of Middle-earth but not everyone knows they are there? Hard to find?

I got a Swordsmith Guild as the Byzantines. It's not limited to Islamic factions.

[Incredible Bulk]

But that's not in TATW?

[Ngugi]

Wouldn't call much of what you bring up like historical people like Marco Polo, guilds or hard to find cities etc easter eggs* as they are part of the era and region of the game, that's a feature(s); however I follow your intent.

The only Easter Egg I know of could be Hross, a name the Dwarves may get, the TWC-name of one of the TATW team members, responsible for names I think hehe.
Sure, it mean Horse in Icelandic and thus considering the inspiration for Tolkien's Dwarven name could fit, but I think it's an Egg.


*

A virtual Easter egg is an intentional hidden message, inside joke, or feature in a work such as a computer program, web page, video game, television program, movie, book, or crossword. The term was coined—according to Warren Robinett—by Atari after they were pointed to the secret message left by Robinett in the game Adventure. It draws a parallel between the custom of the Easter egg hunt observed in many Western nations and the last Russian imperial family's tradition of giving elaborately jeweled egg-shaped creations by Carl Fabergé which contained hidden surprises.