Slavery - this is not Washington where the Pitt organized slave hunts. Nobody really needs slaves here
Prostitution - that would be a pointless gimmick at best
More monsters - the monster rooster is good enough, especially seeing that each is interesting enough. Quality over quantity, DLCs will add more anyway
Sea Monsters - if that means underwater gameplay, than NO
Karma System - Beth discarding this BS is their best decision for this game... not to mention it would kill the Moral Ambiguity that the game aims at
Harder Survival Mode - agree
Aliens - no. Nope. Just no
Weapons - there are many more weapons as each weapon has modifications that can turn it into a vastly different weapon, not to mention that some legendaries have their own unique mod trees
Vehicles - implementing such a feature would mean Beth reducing their budget for other features. And I prefer these other features so no
Choices - the global choices in FO4 are the best in series 'till now and only the side quests could use more. FO3 style of being either a goodie two shoes or a psychopath is so shallow it does not even deserve being mentioned in choice and consequence discussions
All in all, this is a list of things you would personally want, not a discussion of lack of ambition.
The only edgy Fallout is FO3. FO1 follows far after while FO2 and NV are brighter in tone. Even devs of Wasteland 2 (basically Fallout Prototype the Sequel) went for a brighter tone. That is, Fallout is really not about being dark and edgy or whatsoever. Actually, the Fallout Universe is not a story about survival or a harsh and unforgiving world but a story of restoration. FO1 and FO3 are dark because it shows parts of the world that have yet to start restoring themselves whereas FO2, NV and FO4 show a world that is getting back up.