Some civilizations that were considered for The Conquerors during development but didn't make it to the game include the Incas, (because they would have required either an exclusive set of buildings or doing the American one too generic, plus the developers already had difficulties trying to make 2 civs with no horses distinctive enough) and some East European nations such as the Russians and Polish which would have got another unique building set.
Other civilizations that were considered for the expansion pack but didn't make it were the Khmers, Tibetans, Italians, Swiss, Austrians and Magyars (Hungarians). The Italians, Magyars, and Inca finally got added in the Forgotten Empires pack, along with the Indians and Slavs.
The developers considered giving units multiple attack modes, but decided it would add more frustration than it was worth.
An aborted ranged cavalry unit known as "Genitour" survives in the scenario editor. Its graphics were never created and the result is an unusually fast Militia, that behaves like a cavalry unit (clicking on it will even produce a horse sound), has a Cavalry Archer icon and throws the Skirmisher's javelins when it attacks. Common fan speculation is that the Genitour was originally intended to be the Spanish UU before being replaced by the more familiar Conquistador (which, being a mounted hand cannoneer in the game, means that the Spanish get a gunpowder unit - and a very powerful at that - already in the Castle Age, before they even research Gunpowder, but this is also true of the Turks and their Janissary). Others have suggested that it was intended instead to be the Frankish UU (as this is a cavalry-heavy civilization, yet gets a ranged infantry UU unit - and a quite unpopular one - in the Throwing Axeman) or the Mongol UU (who in the final game get two Cavalry Archer units: the common one and the unique Mangudai, who has an extra attack against siege engines). It is most likely Spanish, as it is an alternate spelling for "jinete," a type of light cavalry in the Iberian peninsula.
The aforementioned Turkish Janissary was originally a heavy foot archer◊, before being changed to a heavy hand cannoneer◊, in deference to the Turks' status as sole gunpowder heavy civilization in the original game (they would be joined by the Spanish and Koreans in TC and the Italians and Indians in FE). Then, just before release of the original game, Microsoft insisted that ES took out the Janissary's tall white hat for fear that users would mistake it for a Klansman. Ever since, the first skin with hat has been available to download in fan sites, or included in fan expansions either replacing the hatless skin or as part of a "Royal Janissary" unit available in the editor.
The beta version of the original game had one wonder per building set (Aachen Cathedral for Western Europe, Suleiman's Mosque for Middle East, beefed-up Temple of Heaven for East Asia) like in its predecessor, Age of Empires. Fortunately, the designers took the extra effort to give each civilization an unique wonder that gave them more personality. The Aachen Cathedral and Suleiman's Mosque are still available as eye candy in the scenario editor, and the beta Temple of Heaven can be downloaded from some fansites.
The beta also had different, less detailed graphics for a lot of units, buildings and even map props like rocks. The cliffs, on the other hand, survived unchanged even though they are just scaled-up versions of the ones used in the first Age of Empires, released in 1997.
Like the first AOE, the beta had a single resource drop site, the "Trade Workshop", whose functions were later separated in the Mill, Lumberjack Camp and Mining Camp. The Trade Workshop survives as a function-less, mere eye candy building in the editor, and it only has separate Western European and East Asian graphics (FE adds a version for every other building style, although taking the West European model as basis).