An alternate start mod or one that heavily changes how the game begins (i.e. giving you more creative freedom with your character).
One that brings backs skills. They did it with Skyrim, they can do it with FO4.
A mod that heavily reduces the number of super mutants. I'm so sick of them.
Better looking companions.
Perk mods that include old school perks like Laser Commander, Confirmed Bachelor, Cherchez la Femme, etc (yeah, these were introduced in NV, but I view NV as old school).
A way to renovate your room in Vault 81.
Gives all generic, unnamed characters randomly generated 50's styled names.
More quests that allow opportunities to roleplay.
Better settlement building.
I've a few more in mind.