I bet you it will be Kiwami like challenge. IMO playing on hard mode become less and less fun as each installment are released. Kiwami's the worst offender in this case. In no way does it detract from the experience but i know what to expect from boss battles.
Disagree completely. Playing hard mode for me has become more fun with each instalment. 4-5 especially were hampered (fun wise) because hard mode was far too easy. Ishin, 0 and then Kiwami injected some actual difficulty into fights which made the games far more fun for me. Though it was quite a rude awakening to begin with, especially with Ishin and 0 not giving you any healing items in story fights, meant I had to replay the start of Ishin and almost had to do the same for 0!
And yeah, 6 is sorta like 0 and Kiwami in this respect. In that enemies are actually dangerous and throw a punch. So going into a fight with several enemies and trying to just combo one of them is a bad idea. But some of your combo's in 6 have really decent crowd control capabilities. Plus you have beast style moves for environmental weapons which helps a heck of a lot against the large crowds you go up against.
Bosses though in 6 are easier (at least compared to Kiwami), as most go downn quicker, and even Amon felt easier compared to 0 and especially Ishin where he is pretty fearsome. I forget about Kiwami though, I think he had his moments but if you can counter you can put him down pretty effectively.
Who makes these gameplay choices. In Zero you had to complete a VERY long substory chain before you eventually unlocked it, and then it was expensive as fuck to level up.
You don't need to level the secret style up at all in 0. It already gives you access to all the moves and heat moves unlike the other styles which required levelling. The upgrades were merely to make all styles even stronger and that was mainly benefitial to you if you wanted to do some of the later arena challenges legit or have an easier time against Amon. Plus I don't really agree that the side story chain was long either. I managed to clear my way to 100%ing each area incredibly quickly (I actually slowed myself down so that I'd not complete it too soon, just so I was suitably powerful to finish off a couple of the later billionaire fights who would have been pretty tough if I'd have been under levelled.
Now Kiwami I'd agree with, it forces you to jump through tons of hoops to get that style powered up, but even then its really useful for grinding the fighting arena, beating late Majima battles (Tiger Drop all day) and really fun to use in the last few boss battles in the story.