Thanks to John Carmack and John Romero, Doom was made to last. One community that Carmack respected and loved was the hackers. To that end, he made Doom into an easily hackable game where they can change everything about the outlook and mechanics, swapping out content to make it customized to however you want.
Even if you can't modify the game mechanics, you can still create your own maps using Doom's own components. The only limit is your imagination and thanks to the enginuity of many modders and mappers, Doom has stood the test of time and still remains one of the strongest and longest lasting game fandoms. Doom WADs (or Doom "Where's All the Data" files) can be tampered with, edited and changed to your liking. Just remember to get the creator's permission before you start messing with their maps.
Doom mods like Brutal Doom, Project Brutality, Doom Zero and No Rest for the Living, created their own entities of Doom and freshened up its concept many times over. People hold Doom Mapping contests all the time and one trip through the community will put countless Doom maps and mods at your disposal and enjoyment at no extra cost.
If Doom is your game, you owe it to yourself to try out these different mods. To get into the community, get GZDoom and start looking for commuities with maps and mods. The space taken up in a more modern computer will be minimal compared to the enjoyment you'll get.
Mods like Brutal Doom and Project Brutality up the violence factor, add new and awesome weapons and different game mechanics to the whole thing. It ups the difficulty and also makes blood paint the walls with every shotgun blast. These have breathed new life into the franchise and people have started testing maps on them just to see how they hold up as opposed to just the "vanilla" Doom WADs.
Doom Mapping has become so inclusive that even I have gotten in on the action. I've been Doom mapping for over a year now and I'm still mediocre, but I can make as many levels as I want and play as much as I want as a result. If you're curious as to how my Doom levels are, here's a link to my Doom World post. That's a forum where many Doom creators congregate and show off their stuff. Give them all a look and happy killing.
There are mods that change the game entirely and in just about every way you can imagine. The possibilities are endless. However deep you want to go into the Doom fandom, just be careful, because like any community, there are some dark events and unsettling lore set around the community. Just be sure to check your sources and make sure you know the truth about a lot of it. Rumors and myths have run rampant dating back to its very inception. A good place to start would be the ID biography, Masters of Doom by David Kushner. That will get you started. The rest is up to you. Just try not to go too far down the gory rabbit hole and try to remember to consume the water labeled "Drink Me".
