2) I suspect this is the broad appeal of the Battlemaster Fighter, in which I think the Martial Adept feat is basically this.
I see this point made a lot that this steps on the toes of the battlemaster, so I often reply that the battlemaster is not limited to once per turn, not limited to their own turn, and is not limited to a melee weapon in this regard and this free feature gains no additional damage. The intent is not to eclipse the battlemaster, but to give melee's a sensible choice of effects they can inflict that aren't backbreaking.
I just clash with the idea that "attempting to trip someone" is a limited resource...like why are we making martials like casters? If it's adding damage and being done with a ranged weapon fine, but currently being able to make an attempt to trip with your sword whilst slashing at the legs feels scummy to tie to a limited resource ESPECIALLY when casters have debilitating effects tied to ranged cantrips with no limited resource costs (I'm looking at you chill touch and ray of frost....)
3) This is how I've rolled it on the tabletop, so yep.
I've seen too many monster crits turn a table sour. I'd rather it just be a fixed number that I know won't be lethal.
For many of the class decisions, Larian has said they were aiming for 10th level as the cap, but considered changing it to 12th level to fit more progression in, so many of the higher level balancing is unlikely to make it in game. As a discussion for the tabletop;
Barbarian: As far as increased critical range goes, I think that was the territory of the Champion Fighter subclass, and thus making it partially viable to multiclass between them (though, not really tbh, given redundant Extra Attack and proficiencies).
I have no qualms with stepping on the toes of Champion Fighters. Especially since Barbarians are pigeon holed into melee for a majority of their features. This limitation becomes especially apparent in tier 3 and 4 when ranged and flying enemies are a dime a dozen. Anyway, we can agree to disagree; I think Barbarians having more opportunities for their critical damage feature to shine is important for player morale. Like "cuss yeah this is why I rolled barbarian!"
Berserker: Missing out on one weapon attack seems like a low priority to me (though doing damage earlier is technically better than doing it later, sure). If your average Ranger/Warlock has to burn a bonus action to cast or apply Hunter's Mark.Hex, then I think the bonus action defecit is okay.
I think that this is a bit of an apples to oranges comparison. When I look at and analyze the frenzied barbarian it is up against the other barbarian archetypes (not rangers and warlocks), with the primary contender being the totem barbarian.
Mechanically it has less to do with impact and more with player morale; you frenzy, and the benefits of that frenzy would now come online right away because you don't know how long you'll be able to keep the rage up (because you need to take damage or make attacks) and so you are immediately rewarded for the penalty you will be taking later. If the combat closes out early, or you get knocked out of rage you get at least -some- value.
Fighter: I do think Fighter get the short end of the stick as far as martials go. That said, getting back every use of Action Surge, Indomitable and Second Wind with each initiative roll is ... a bit much. Whether a classes capstone should be "you get back resources when you roll initiative" or the more unique ones like Cleric, Wizard, Barbarian, or ... *shudder* Druid, is probably a better consideration to have.
Ehh, it's mostly just trying to play into the Fighter's strengths. They are about sustainability and "keep going" and most of the time, you can get that short rest you would need anyway, this just makes it more convenient for the player. The DM is largely responsible for how accessible short rests are in the first place, this just takes that lever of control away. Ultimately you can still have longer fights that tax that system.
Arcane Archer: A decent buff, but one that probably incentivises small multiclass dips rather than a pure class. I'd probably have it scale off Intelligence, which is slightly palatable given Fighters get more ASIs (though it does compete with Sharpshooter/Crossbow Expert and Dexterity).
Doesn't that incentivize multiclassing too? Like you have a stat that you can boost for another class and still get lots of magic shots? And it frontloads the feature moreso since you can get 20 int well before level 13. I suppose it has a bit more of a character build cost but I tend to deviate from MAD character design as much as possible.
Monks: This is a bit tricky, because constant Ki means constant Stunning Strike. The most I could grant is you can do a small meditation outside of combat (recenter your Chi, that sort of thing) similar to how they made Prayer of Healing an instant out of combat thing. You'd have a pool of this meditation Ki equal to your Wisdom. So you can easily start each fight with more Ki, you can burn through your per level reserve (which you'd probably have to halve to compensate) and still have room for the Way of the Four Elements to burn through Ki, value their Short rest, etc.
As for free Flurry of Blows at 20th level, I'd probably just double the damage die of unarmed attacks and leave it at that. Even allowing for 4 Ki when your per level Ki is below 4 is still decent.
I have grappled with the idea of a mixed system where you have a small per round ki pool and a reserve that draws from wisdom or proficiency bonus and it just lacked elegance overall. 5e seemed to strive for simplicity so I went with what I felt was both simple but effective (I have tested this change with many monk players and they all had the option of not going with it or going back at any time and they never looked back, what's more no other players at the table felt they were overshadowing the other players at the table; stunning strike? woopty doo, the wizard has been incapacitating 5+ opponents for a minute since level 5)
Monks in tier 3 are still making big choices; do I want to flurry, and smash out 3 stunning strikes every round, or am I going to spend my bonus action dodging, get only 2 stunning strike attempts. What's more, when you spend Ki as part of your reactions (like Deflect Missiles, or Diamond Soul) you now have to factor that into the way you spend ki on your next upcoming turn. Perhaps a mixed system would be better. I will do some more thinking.
Rogue: Crippling Strike feels like an entire subclass feature. Better yet, replace the Assassins lackluster Imposter and Infiltration features with this (though maybe replace one of them with at will Disguise Self).
Assassin: Depending on how its implemented, Assassin would be very powerful in this game, mostly because of how stealth works in this game. Going on a shove spree in this game is fairly easy, and I can only imagine a critical hit spree (plus if critical hits do one die of max damage) would be quite effective. Larian seems to have padded enemy HP to make one shot kills less likely, but given how accessible rests are, its mostly a setup to make us burn resources.
Surprise mechanics are a bit weird. The changes to assassin were mostly a throwback to how shitty they are in tabletop.
As for crippling strike, it -was- originally a level 9 assassin feature I made that had only one negative side effect that could stack; you land a sneak attack from an attack made with advantage and the target loses half their maximum speed. If they are affected by the feature a second time and fail their save their speed is reduced to 0 and they drop prone. They can keep attempting the save at the end of their turn to regain their speed. It was a clutch homebrew that saved the subclass for me but I needed a tool to introduce decision making to rogue characters and unlocking the feature at level 5 in this way gave them just such a tool that lined up well with other martials getting extra attack. The intent is to give martial characters decisions to make. I wanted something that felt impactful and team oriented without being too strong. This seemed to tick off the right boxes.
Sorcerers/Warlocks: I suspect this is an unpopular opinion, but I actually don't think either should get extra spells than what the PHB outline. I actually like the idea that Sorcerers have limited natural spells similar to Bards, and that inherently some classes should have less spell flexibility than others. Wizards should clearly not be able to copy any spell list via scroll, which should probably be a feature of the Bard (limited to Magical Secrets, if we ever get it).
Overall though, I'm okay with of these as tuning changes, just as long as they're all individual choices.
Warlocks have many tools to supplement their lack of spells known which is why I don't bolster them. Sorcerers have only metamagic which is already a derivative of their spells known. I've played Sorcerers extensively. They require an abhorrent amount of min/maxing to play well and the gaps they have from lack of spells known make them wholly unattractive to the average player; I intend to alleviate that.
Thanks for the feedback, hopefully I changed your mind about maybe one or two stances you originally had.