Evil Outsiders don't really need the boost, I agree. However, no Ranger ever takes Lawful or Chaotic outsiders. I'm fairly sure that if every non-evil-outsider Outsider was lumped together, Rangers wouldn't take that either (Although tieflings, mephits, and the occasional djinn/efreeti wouldn't be bad, just wouldn't be as good as other options). If we decide Evil Outsider shouldn't be any more inclusive, I don't have a good suggestion for the leftovers.
I think non-evil Outsider as a category would be worth considering for Djinn, (effreeti are evil in all cases I think), Mephits, teiflings are prevalent, and occasionally difficult.
One of the things i was thinking when i types this up was "This might be a bit much for one category". I see you agree with my internal monologue ^_^. I like Gnolls, Gobs and Orcs getting their own category, but I would feel like that would still leave Monsterous Humanoids and "the New Goblinoids" as both mediocre. I really wanted to spice up monsterous humanoids a bit, since they seem like a contender for the coveted "Top 5" for Ranger Favored Enemies... but unless you run a lot of VoN/Fens Epics, they just dont' quite make it.
I think minotaurs, sahaugin and gargoyles covers enough of the endgame (and much of the leveling process) to be attractive, while gnolls are everywhere, and orcs and goblins are prevalent enough to make the whole category worthwhile.
Still feels Sparse to me. And again, seems like something very few, if any, Rangers woudl take a second look at.
Kobolds and trogs are prevalent for the entire game (remember that fiendish version count), though there aren't many in epics, and few are difficult, and dragons are worth hurting badly. If we had more dragons at endgame (WHERE THE **** ARE THEY?!) this would be a strong category, I think.
Now this comment i don't understand at all. While vermin, animals, and magical beasts are all common, they are never more than a minor inconvenience. I actually feel like this category is a little light. Having Magical Beast as its own category (Which includes what... Worgs and Frost Wolves?) would be as unusable as it is today.
For some reason I had thought there were more in this category. Nevermind.
True, Elementals aren't extrodinarily common, but they are common enough and difficult enough to fit into the "Top 5". Mephits was more of a thematic combination, especially since Mephits are more of an Annoyance factor than a real threat. Djinn and Efreeti were just there because they are incredibly uncommon. I think Elementals and Mephits would be nice, but i can see both sides of the argument.
Maybe add plants here?
It is dense, but only due to the rarity and relative harmlessness of most humanoid enemies. Deurgar and Drow are the only really difficult bits of this package, and are lumped nicely with some semi-common fodder enemies. Again, I was shooting for a "What will make this group attractive enough to be a good alternative for a Top 5". I'm not sure having all of these together as "humanoid" would make my Top 5, but it would get close. Having human, semi-human, and "tiny" wouldn't even get a second glance from me.
I feel like drow and humans appear in enough content that they could maybe stand to be separate, though I'm trying to think of difficult bosses of either type and coming up blank.
After a bit of thinking, i think Plant and ooze would work well with Abberation.
Ooze, yes, plant, no.
I agree with your thinking that there can't just be 5 favored enemies that cover everything, but i disagree with your breakdown. I believe each favored enemy category should be held up against something like Favored Enemy: Undead, and if it doesn't compare well, it needs to be reworked til it does. Undead, for example, has: Enemies in all levels, found commonly, with a mix of tough enemies, easy enemies, complete fodder, and bosses. Some of your suggestions would still leave categories that don't hit many of these marks (Human, ooze, magical beast, etc), while leaving the dominant Favored Enemies still dominant. My suggestions aren't perfect mind you, Beast is still something no ranger would pick up, and Reptilian still feels lacking to me, but I feel they're a step in the right direction.
As for the count, there are several big categories i didn't even mention. If my plan were followed exactly we'd have: Humanoid, Monsterous Humanoid (not including Plant), Beast, Undead, Abberation (Including Ooze and Plant), Giant, Outsider (Or Evil Outsider/Other Outsider, since i do agree Evil Outsider probably shouldn't absorb all other outsiders), Elemental, Construct, Reptilian. I feel like I'm forgetting some, but thats 10 right there, and 11 if we split up the outsiders. This would still leave rangers with 50+% of the game off their favored enemies list, but would ensure that they see plenty of favored enemies, and that their favored enemies are everything from the scrub they wade through in mass droves, to the big tough enemies they fight two or three at a time, to bosses.