New California is going to have a raider faction that makes chainmail and what look like primitive shields.
I'd love for enemies to have to use torches or flashlights, or be blinded in the dark, other than of course creatures that would be adapted to night hunting.

How about vampires.
Could be a mutation

Swords could very well be a major item in Fallout as well. Useful for defense in close quarters and against feral ghouls as well. Anything to save ammo which would be hard to obtain and quick to deplete otherwise.

Some of the construction. Small stone buildings like found in Skyrim (slightly different style though) Being the 20th century homes really would be in poor shape. Also stone buildings would be much more protection. Notice the wooden shutters on the Skyrim buildings as well? Good to help keep the occupants safe.

Can't think of anything else right now.

Enchanting of some form would be nice. FO4 incorporated it somewhat with Legendary effects.....but most of them are just garbage and not worth it. Having actually useful enchants and a way to craft them would be good. Not saying to incorporate magic or anything as that would really be going too far, but the mechanic of making your weapon(s)/armor(s) more unique/effective would be good for FO4. In vanilla FO4, you're stuck with just finding legendary gear that you have to settle for rather than actually making it.

Potion brewing would also be a good feature to bring over. You can craft chems, but they are all the same really. Being able to make homebrew chems with different effects could spice up a game. Brew up a speed chem to run a bit faster or one to make your jumps a bit higher.

As others said, I think a form of shield blocking would be a good feature to incorporate. Same with dual wielding melee. It makes no sense why you can't pick up two knives or why nobody would think to pick up a trash lid as a makeshift shield.

ES could really benefit from FO4's settlement system. I have little doubt that the next ES will be lacking in a lot of areas so a settlement system would definitely help with the appeal.

ES could definitely benefit from crippling limbs, doctors, and barbers. ES (at least Oblivion and Skyrim) lack any real body damage so getting smacked around by a giant and suffering no broken bones doesn't make any sense. Having FO's criplling system, it'd help spice it up a bit. Doctors would also be nice. Yeah, some towns have mage healers, but are often not worth the visit. With crippling, it could make them more useful and non-mage doctors would be a small nice touch. Barbers/surgeons are pretty obvious. I didn't enjoy having to do a few side quests and pay some bitch 2,000 gold to shave my beard in Skyrim. Having surgeons and barbers more common would allow players more freedom to customize.

FO could also benefit from ES armor slots. I wish we could have hat/top/pants/boots for FO. There are bits and pieces of armor/clothing I like and dislike in FO. Being able to mix and match would be nice.

For ES, it would be interesting to see if they could incorporate some form of power armor. Maybe a really OP bulky armor that needs some kind of energy to run. Not scif, but more magical.