One such feature I would like to see a return in Fallout 4 would be an improved Hardcore Mode from New Vegas.
The system would delve deep into the essentials, I.e several meters to monitor your sleep rate, fluid intake, and starvation, as well as adding weight to your ammo and making Stimpaks less effective.
The mode could also improve on like the "Super Hardcore Mode" mod developed by Obsidian's Chris Avellone (Who also worked on Fallout 1 and 2).
Obviously, for anyone that doesn't know, this feature would be optional.
Link Provided to the Mod Description:
http://www.ign.com/boards/threads/new-super-hardcore-mode-mod-released-for-new-vegas-by-obsidian-itself.250006429/
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Why not, it was in fallout 3 and new vegas, if you didnt want to you didnt have to use it and if you wanted an extra challenge then you could use it. I am always for the increasing of our options so that everyone can enjoy the game how they want.