OK. Actual feedback:
First, I'm not sure how you figure that ranged combat is less effective than melee combat. It's true that the Rogue's sneak attack is very potent, but that's a completely different conversation than ranged v melee. I'm actually inclined to say that the issue is with sneak attack and that the bar should be somewhat higher for using it. Rogues are largely skill monkeys and it might be more appropriate to encourage them to behave that way, rather than inviting a serious conversation about whether Rogues are better at filling the swashbuckler and/or William Tell archer archetype than some other class.
Granting sneak attack to concealed ranger characters just like the rogue.
I wouldn't do this. First, as I said, above, I feel that sneak attack is still a bit too good. Spreading it around just aggravates the problem.
The idea, itself, raises questions about what your goal is with the Ranger, though. What is a Ranger, to you? There are more than a few folks who'd be fine with turning the Ranger into a Rogue kit that got some bonuses in the woods. There are also a number who'd be fine with making the Ranger a Fighter kit. There are other options, too. A common criticism of those who advocate for a rebuilt Ranger is that some fans seem to want the Ranger to be better at Fighting than the Fighter, better at sneaking (at least in the woods) than the Rogue, have more hit points than a Barbarian, cast spells, and still have a few tricks of their own.
Personally, I've settled on the idea that the Ranger's niche is really to be one hard SOB to kill -- not because of hit point (though those help), but because they do a very good job of using their surroundings. Yes, an 8th level Ranger (mid-range PC) is going to be better than fighting than a city guardsman (3rd level Fighter, at best) and better at sneaking than a journeyman guild thief (3rd level Rogue). It's just fine for the Ranger to be second string in straight-up combat compared to the Fighter (or even the Rogue by current niche). If you use the latest UA Ranger and they actually cast
hunter's mark, they can be pretty effective. I'm pretty sure the (archer) Ranger in my current game has the highest kill count besides the Moon Druid (which is crazy, so far).
Granting ranged combat specialists their proficiency die as additional damage.
Define "specialists". If you mean a character who takes that combat style, it could work. I don't think I'd combine it with the proficiency die option, though, because of the critical hit rules. If you do it, though, it should be a replacement for the current +2 to-hit, not additional.
If you mean something else, I'd like to know what mechanic you're using. Assuming it's a feat (or something else modular), one should be able to specialize in melee, as well -- especially if you're interested in "realism" (or just verisimilitude). The benefit should be on par with what's granted for ranged specialization. So... no net gain.
Granting automatic injury to the target on a ranged attack that hits with a die roll of 25 or higher.
What the heck does this even mean?
Whatever it means, it breaks the general mechanic that attacks either succeed or not. The die roll doesn't matter, except for the natural 20 crit. Whether it was a good hit or not is determined by the damage dice -- that's pretty much the whole reason they exist. I would not add a 25 or higher tick.