The Fixt mod, though still somewhat buggy, offers an interesting insight into what the developers originally had in mind for the game, restoring many features that were either never properly implemented (the good endings for the Hub and the Followers) or patched out due to being unpopular (towns slowly being overrun by the mutant armies as time passes, eventually ending with Vault 13 overrun after 500 days). Some other oversights were also addressed, making useless skills and traits (like Traps and Skilled) have more of a practical purpose. I say check it out if you want to see the game as its creators intended, or just get a more complete Fallout experience.
If it is your first playthrough, I advise turning off the mutant invasions at first, or at least setting the timetable to a higher value; the invasions rob you of nearly all of the quests and shops over time, and any good endings you may have earned in an area will be nullified when the Mutants come in. Getting all of the good endings and not letting any towns fall can be considered something of a challenge for die-hard players, as they must carefully manage the time they have before the invasions begin.
(Or if you're really nuts, try the option that has all of the towns invaded right off the bat and Vault 13 going down 125 days in...)
If you're curious, here's the two endings you couldn't normally get in the base game. Time limits are based on the "Version 1.0" settings.
- Good ending for the Followers of the Apocalypse (Fixt requirements: Nicole and at least 12 other Followers alive, the spy in their ranks is dealt with, Master is dead within 90 days)
- Good ending for the Hub (Fixt requirements: Harold is alive, Decker is dead, Master is dead within 140 days)
I mentioned an oddity in the worst ending for the Brotherhood, so I'll elaborate a bit more: The worst ending mentions a traitor in the Brotherhood's ranks. According to the Fallout Wiki, this was supposed to refer to a character named "Kedrick" who would have been a Unity spy planning to assassinate the elders; the player would have to discover proof of his treachery and expose him to prevent the bunker from falling. However, neither the character nor the quest were ever included in the game (and remain unimplemented in Fixd), leaving the traitor's identity ambiguous.