Hiya.
I can't see what's wrong with the Ranger - Beast Master. As far as I can tell, it doesn't need any changes. Hmmm...
The Good
The Ranger doesn't need to acquire higher CR creatures. Adding the proficiency bonus to...everything...and using the new HP calculations means that the companion isn't irrelevant.
This adds HP to the party. Without taking any actions, the ranger can stick their companion in the line of fire and use it like a meat shield.
Compared to a weapon, it's viable, and more flexible. Because its attacks take the place of your own, its worth comparing this to the other weapons you could use. An animal companion does decent damage when compared to most martial weapons (especially when you add your proficiency bonus to damage), and can do this damage at any range. It could also do things other than damage.
It can do things for you. Need a trap-filled corridor traversed or a switch flipped or a vial snatched? Animal companions are good for suicide missions, or when you need to hit something across a room. Flying companions are especially good at this.
The Questionable
It does nothing unless you use your action?. A mount can move or dodge. A familiar can help. Compared to that, a companion just stands around drooling unless you spend time to make it do something. A familiar doesn't add as much HP, and doesn't necessarily scale well, but it can do things for you AND use the Help action. A mount might not scale as well, and it can't attack, but it's a bunch more HP for the first few levels, it's replaceable, AND it can Disengage for you. Companions don't seem as good as just buying a friggin' horse or conjuring up a celestial badger, at least for a while.
It has to be Medium or smaller?. I WANT A PONY. This is likely based on a "you should be able to bring it into a dungeon" requirement, but iconic animal companions are often larger than medium. The mountain man befriends a bear, the outrider befriends their horse, etc.
Compared to the Hunter's abilities, this is no great shakes. Colossus Slayer ups my damage by a d8 almost every round. Hoard Breaker and Giant Killer give me extra attacks under certain circumstances. They're pretty context-dependent, and they don't up survivability or increase utility like a companion, but it's a little weird that having a panther claw and bite for me won't actually up my damage very much.
Is this meant to be an animal friend or just some expendable beast?. The ability to acquire a new one implies the latter, but the HP totals imply the former, and the archetype certainly contains "me an' this bear were raised together!" kinds of narratives.
Your "Good" seems to imply you only see the animal as a data set of numbers to be used against other numbers in a game of numbers. This...is....odd? I'm not sure what words I'm looking for. I guess I'm sort of equating it to a father or mother looking at their 5 year old daughter and only being concerned with how she can be used to further their own wealth or career. "Hmmm...listen, Suzy, why don't you go over to that tiger in the tiger exhibit there and pull it's tail. It's ok...mommy and daddy will get lots of money for your death. We can always have another child". <-- ok...that was
REALLY harsh! But I'm trying to make a point. I guess the point is...you should probably just stop reading this post right here if you have no problem with that scenario in terms of an RPG situation.
Right. So... With regards to your "Questionable" parts...
It does nothing unless you use your action? Er, I suppose...if your DM is a complete and utter twit. This isn't a video game or a game with cold, hard, immutable rules. It's a roleplaying game. I'm pretty sure your animal companion would do whatever you would expect an animal to do...unless it was told to do something else. I would assume that the beast has some kind of affection for you, so I'm also pretty sure it would attack anyone attacking you. That said, if you were to command it to do something else, it would. But, lacking that command, any DM worth his salt would play the beast as a loyal, protective, and loving companion. Of course, if you just see it as a "bunch of numbers to be used in combat", and so does your DM...well, I guess that's on you.
It has to be Medium or smaller? As per the rules, yup. *shrug* I really have no comment on it other than to say that this would fall squarely on the "Hey, I have an idea...can I [insert cool idea]?" players shoulders. And then firmly into the DM's court of "Yeah, I can see that. Sounds cool. Go for it!". But, again, if you just see everything as a "list of numbers to be exploited in a RAW manner, as best as possible, and totally for combat purposes"...well, again, I guess that's on you.
Compared to the Hunter's abilities, this is no great shakes. Yeah, but...well... *sigh* Colossus Slayer can't help you detect hidden foes or fly overhead to try and detect ambushes. Hoard Breaker and Giant Killer won't help keep watch during the night, nor will they provide warmth when deep into the bowels of the earth, and no, they won't go running off to go and try and find help to bring back to your unconscious (and slowly dieing) body. Any other combat/damage-based Feat/Ability you toss out there is only 1/3 of the 5e pie. Animal companions have FAR more benefits than how much DPS they can bring to bare. I mean, I've seen so many fantasy movies where the "animal companion" is used for comic relief, or to pull on our heart strings, or to otherwise show all the stuff that being heroic and fighting evil is all about: saving or helping people and making their lives better. But, again...if it's all about the "combat numbers", ...yadda yadda yadda.
Is this meant to be an animal friend or just some expendable beast? Really? I mean, seriously really really?
Uh, I'm going to go with 'the former' due to the fact that it's kinda the archtypes KEY ABILITY/FOCUS/SCHTICK. Would it make more sense for this archtype to suddenly loose it's key thing
forever? That's like equating a barbarians Rage to a can of spinach. If the barbarian ever looses that can of spinach (re: it gets disintegrated, lost at sea, stolen by thieves, eaten by a grue, etc), the barbarian forever looses the ability to rage.
...Holy Curmudeonly Grognarditis Batman! I just reread what I wrote. I guess I'm definitely in a snarly mood or something. I'm gonna leave this here before I *really* say something I'll regret.
^_^
Paul L. Ming