Dummied Out: If you know how to use the Level Editor, you will notice that there's a lot of content which didn't made the cut for the final game.
The QuadShot, for instance, was supposed to be a quad-barrel pump shotgun with the ability to load all four barrels before discharging. It was probably decided to cut it for game balancing reasons - as becomes evident if you install a Game Mod that restores it, four shotgun barrels worth of hitscan pellets are a guaranteed one-shot kill against anything short of bosses. It got replaced with the less unbalanced and more setting-friendly Flak Cannon.
What Could Have Been: While the retail game seems fully featured, Unreal also went through many complete redesigns and had countless amounts of cut content.
There were some deathmatch maps which were cut from the final game in Unreal, but appeared in the betas. One of them (Morbfanza) was a mashup of three other levels which did made the cut. (Morbias-Ariza-Deathfan)
A part of the Expansion Pack is made up of a fraction of maps originally cut from the actual game, among them, "Nagomi Passage" (a Hub Level) and Velora Pass, which in the betas was named Aztec.
There was also a Playstation version with a completely new campaign that was cancelled and never released despite being nearly finished.
The expansion Return to Na Pali originally would have been completely different from what we've knew. It would have featured the Skaarj and Mercenaries as playable races, with exclusive weapons, and a same level could feature different enemies/allies/goals/items depending on the playable race.