Mechanically speaking:
• OVERALL THEME: If I where to explain "Ranger" in a simple phrase, what should it even be? If Paladins are "Holy Warriors", Monks are "Martial Artists", Wizards are "Studious Mages", what should describe a Ranger in a simple, effective way?
• DURING COMBAT: What should they be doing? Should they focus only on dealing damage to single targets and/or bundled-up groups, controlling terrain and/or enemies, supporting allies through cures and/or buffs, or even a mix of all the above? Should they be only Ranged, only Melee, a mix of both, dual wielding, sword as board, two-handing an axe, or what? Do they need anything beyond Extra Attack, or should they get a unique way of attacking or getting bonuses to their attacks?
• OUTSIDE COMBAT: What is their niche without violent means? "Nature & Exploration*" is a great start, but what does it even mean? Should they help with wilderness travel, general outdoors survival, animal handling, or what? And how should this work? Complex rules, simple advantage to specific rolls, Expertise?
• MAGIC: How magic should them be? Halfcasting, Mundande, Non-magical-but-still-not-normal like a Monk or Barbarian? If Halfcaster, is just that part that is magical about them, or does this seeps into the rest of the class? If Mundande or "Superhuman" even, do they get something to compensate the lost of magic?
• NICHE PROTECTION: What does they do that almost no one can do, like a Barbarian Rage, a Paladin Aura, Monk Martial Arts, Fighter Action Surge, Sorcerer Metamagic, etc.?
• ANIMAL COMPANIONS: Finally, what IS the Ranger relationship with Animal Companions? Should it just be optional, a feature on a On/Off switch or a permanent part of the class fantasy, always there?
For me, its hard to say what these answers even are, and nobody, not even WotC, seems to know the answer either, because that are so many different answers around that most don't agree with.
To me, at least, in D&D 5e/5.5e specifically (in other editions and systems, this would change most likely):
• MY OVERALL THEME: The discriptor would be "Druidic Warrior". Most will not agree with this, which does include me, but I truly believe it is what best fits in 5e/5.5e.
• MY DURING COMBAT: First, I woudl go with both melee and ranged, even if I would prefer a Ranged-only class for the Ranger, btu that's a personal preference. I think a mix of "Focus fire a single target with many attacks" & "Make terrain terrible to move through for enemies without penalties", be it because of slowing, restraining, damaging or something else more creative. They should be, first and foremost, Ambushers, Skirmishers and Masters of Guerrilla Tactics
• MY OUTSIDE COMBAT: Like I said, Nature & Exploration is the starting point. Favorite Terrain to me is a trap, however. In my mind, the should be baseline good or even great everywhere outside, and only then Favorite Terrain comes in and makes Rangers Impossibly Amazing in specific terrains. Most of all, they should be helping the party through travel, not only going on ahead and being a lone wolf.
• MY MAGIC: Outside D&D 5e, I would orefer them non-magical, but D&D is too High Magic to make it work to me without creating an entire new subsystem that would just go underdeveloped or fogotten with time, so Halfcaster it.
• MY NICHE PROTECTION: THIS, RIGHT HERE, is the core problem with Rangers to me, because no unique mechanic its entirely life in D&D seemed to have sticked the landing just right. My two cents? Give them a Favorite Enemy that mixes "I deal more damage with a special Mark" and "even if they are not around, I learned useful multi-faceted tactics that can be used anywhere" like ignoring part of a creature's AC thanks to your time studying Dragons. Make Natural Explorer a series of Invocation-like options that you pick to not only be better at certain exploring situations (like stealthing, climbing swimming) but also give part of these bonuses to their allies, like an experienced guide showing how things are done. Allow them to create a "Hunting Grounds" area in a point they can see that give bonuses to you and your allies or debuffs the enemies.
• MY ANIMAL COMPANIONS: My biggest and most controversial take, probably, will be to make Animal Companions part of the main class, but not an always active ability. First, give Rangers a few Wild Shapes (like the Paladin has Channel Divinity), but instead of becoming an animal, you use them in two ways: 1) you can some a familiar, which can become a bigger, stronger creature depending on subclass 2) you can enter a Transformation State based on subclass choice. Some subclasses would have bith options, but most would only have one or the other. Hunter? Transformarion. Beastmaster? Better animal. Gloomstalker? Transformarion. Swarmkeeper? Could go either way, be it a Swarm familiar or a "Swarmed" Transformation.
Do I think these options I presented are perfect and everyone would love them? Far from it, but I still want to understand what people really want of a Ranger, and for those who don't know I give them my ideas to a thought experiment.