The beastmaster is a totally different issue, and I think Wizards is going in a doomed direction with the concept. A spirit beast that manifests to fight once per day couldn't be further from what my players want as a beastmaster. I believe anybody who wants to play a beastmaster wants to have a true animal companion. If it makes the ranger a little overpowered, so be it, because by now we know that a beastmaster that is just a little underpowered is not making the cut.
If I were Mike Mearls, I'd just bring my team together and say: "You're professional game designers, give me a balanced ranger+animal companion build. If you can't, at least give me one that people are willing to play."
I suspect people who pick Beastmaster are most likely trying to recreate their Hunter from years of playing WoW. And the class... probably should respect that - because that's a very enjoyable playstyle for a lot of people (I was never really one of them, but it probably accounts for half of all WoW subs).
WoW, Guild Wars 2, and a number of other MMOs have all created versions of this playstyle. There is no reason it cannot be done right in a table top RPG...
Some basic 'theorycraft' not yet thought through:
- Allow Rangers to amp up the kind of animal they can train as they level. And the pet they have gains in power as they level.
- Allow them to give the animal a fighting style - offensive, defensive, or agile. The animal would then get a few tweaks to its combat based on the choice.
- Allow for MMO style 'commands' - command your "pet" into guardian, 'passive', or 'guard that' modes. In guardian mode it protects you, an attacks anything you attack. In passive mode your pet is away from the keyboard getting soda for this fight..., in 'guard that' mode you pet moves to guard someone or something an attacks anything that attacks that thing or tries to 'handle / take/ break / mess with' it.
- The pet, after that, is either DM played or the player can control it within those commands if the player is the sort who can be trusted to not give it a magical ability to come up with new commands on its own...

- Changing the command style would be an action.
- these are just the basics seen in any MMO pet class. All of it lacking numbers so it could be balanced to a table top game with relative ease.
2) Accept that a combat pet is inherently unbalanced, and that a balanced pet will be unfun. That is, design the animal companions for those who are willing to give the beastmaster more than her share of the spotlight; and tell those who don't that they don't have to allow the subclass.
2a) The animal companion absolutely must pass the fireball test: any level-appropriate area attack should not cripple the pet; or it becomes a liability, not an asset.
2b) The animal companion absolutely must pass the WTF test: being given the same freedom to act as any other NPC, summoned creature or familiar.
2c) The animal companion absolutely must pass the emotional attachement test: the class needs the tools required for the AC to stay alive as long as its Master stands. Class design must NOT expect the AC to be discarded and replaced.
10 years of playing MMOs in the time I've been away from table top gaming informs me that no, its not inherently unbalancing as a concept. You just need to actually design for its being present when that class is around.
Of course some of the MMO gimmicks used to make it pass those 3 tests you have there might raise some eyebrows with table-toppers. Specifically moves like the WoW patch that caused pets to only take 10% of damage from any area attack. And the ressurect-in-combat ability pet classes in both WoW and Guild Wars 2 have with regards to their pets. But other solutions can be found for the same problems.
2) benefits that is incompatible to D&D as a group activity. Example: "when traveling alone, you can move stealthily at a normal pace". Anything that turns the game into a one-man show is bad. Solo mini-scenarios such as "you'll stay behind while I scout out the mind flayer camp to find the princess" have no place at my tables.
Yep. Anyone who has ever played any of the various Cyberpunk-genre RPGs that were around before the wider public got internet access, ie, before the mid 90s... knows this all too well. Everytime the character that was the 'decker / hacker / whatever we're calling a person with a web-browser and dial-up this time' turned on his Commodore-64-equiv-PC and went 'Tron-Mode' all the other players could basically go out for pizza. Not call delivery, but just take off and come back in a few hours... because the game was now on hold (especially in the games that made typing on a keyboard into a 300-baud modem 10x faster than reality for... reasons... so your hacker could play out an entire campaign before the other players got to lift a single plastic caltrop and throw it at you for letting that guy play that character, again...).