I'm a really big Fallout fan. A few of my earliest posts were Fallout related. You can read them here and here. My initial idea, which never really came to fruition, was to use classic unisystem to play the game. However, 5 years later I think the newest edition of Dungeons and Dragons would be the perfect system to hack and make a Fallout tabletop.
For character creation players will pick race and background. Like some other post apocalyptic table top games there won't be classes. I think the proficiency bonus system of 5E works well for a classless game. I will need to modify skill selection, but that shouldn't be hard. As players level up they will pick perks (which will work similar to feats) based on perks from the Fallout games. I'm going to take a look at the DMG for firearms and future tech rules. Weapons will probably be lumped into broad groups like the games. Monsters are easy to stat up in 5E and some I can swipe directly from the Monster Manual.
I'm not sure how I'm going to handle radiation. It will be an energy type obviously. I'm thinking it will be like level drain in 5E. Once you're hit with it, your max hit points are lowered until it's healed. The one to one ratio sounds really harsh though. I'm thinking every 5 points of radiation damage, or maybe every 10.
Here's a preliminary look at what I'm thinking.
Super Mutant (standard and nightkin)
Robot (Protectron, Android)
Sort of like feats. Not sure what level interval characters should get them. In the original games you received one every three levels. Fallout 3 gives you one every level. New Vegas gives you one every other level.
So this is a rough outline of what I need to stat up and my thoughts. Any suggestions by other fans of the game or system would be appreciated.