No one ever, ever, ever considers features that have yet to be added to balance this shit out. Ever. As things stand there are two features that I know of off the top of my head that are going to get added at some point that will significantly increase the difficulty of the game:
Functional NPCs
When in, I can only make assumptions here, but as I understand it NPCs will work together to cause problems (Bandits) as well as clean out towns before you get there.
Roaming Hordes
Again, only assuming, but it’s entirely possible that encountering these wandering hordes will but a huge dent in your ammo supply, or at the very least require you to make an unplanned move.
Onto the spooky scary future guns. “Everyone” seems to hate them right? I’ve heard a few people complain about them but the complaining seems to be very limited in actuality. I’ll admit, they’re a bit common at the moment, and frankly the item spawning system needs some work. Yes, these guns shouldn’t be so common, however they shouldn’t be removed. Yes, these really shouldn’t spawn in gun-stores, personally I feel they should spawn on extremely rare occasions in gun-stores. (Think Artifact rare.) But semi-commonly on something along the lines of a squad of either living or dead PMCs.
“Dead Rising” Weapons are a subject I’m afraid I’d have to agree with you on. Though not as widely, the assorted flame swords, flame bat, the “Combat chainsaw” and whathaveyou are more than a little unbelievable and out of place. The Spray Can Flame Thrower however isn’t too out of place, any idiot with a can of Axe and a Lighter has at some point giggled their ass off when they started spraying the aerosol and lit it. However it shouldn’t be an effective weapon against zeds, maybe NPCs and Animals which are affected by fear.
Are you aware that most the “super-mega-death-win-game-mobile-base-of-destruction” vehicles that are legitimate take months to build, and a great deal of them are not legitimate. So as far as “balancing” those goes, short of disabling the debug/cheat menu you’re kinda SOL. The only additional “balancing” that I think could be done:
Some balance work to solar; the panels themselves should be considerably weaker, but more common on solar vehicles. Currently a single 4 by 4 foot solar panel seems to be able to power an entire car. “Because the future” is a bit much in this case. On top of this, reinforced solar panels, which have extra ‘crap’ in the way of the solar cells, are just as powerful as unreinforced panels, which also probably needs nerfed.
Electric Engines are considerably more powerful for their weight and size over gas-powered engines, as a result everyone and their sister uses solar everything, when the game itself is making it obvious it’s not that common.
Plasma Engines are weaker than most engines, yet use the rarest fuel. I can’t help but find this a bit odd.
Removal of all the “Foreign Things” …That uh, sounds more than a little xenophobic. It’s worth mentioning that these weapons and armor don’t spawn on every corner, they spawn in mansions for the most part. They occasionally spawn on gangsters, though it’s very rare and even then it’s only a Katana or… japan dagger? The armor, to my knowledge, can only be made, which is a very late game skill.
TL;DR: Difficulty increasing features need to be added, things don’t need to be removed.