Hehe, dialogue? Well, you are thinking of DoomRL as ADOM I suppose?
You start on Mars surface, you can wander from location to location. You have to follow the orders from High Command from Earth (since there is noone alive (really?) on Mars except you). At some point, you could join the dark side, turn into some hellish mutant, get granted with new powers (hurling fireballs, spitting acid). Your task would be to wipe out all remaining human forces on Mars.
But there also would be a third alignment, Aliens that ones ruled Mars. Hey it's the official story behind Doom 3, right?
Implementing such stuff (maps, plot, characters, new weapons, powers) would be really, really difficult, and there is only one men behind the project. This would require a bigger effort, from many people so such game could evolve.
In the beginning, thinking up a plot would be really difficult. What is UAC really up to? Why the hell forces want everybody dead? What are those aliens? What do they want, etc. etc.