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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I wouldn't use it, but more options in a game never hurts.