From what Quin has said, and from observations of the setting to date, these appear to be the non-negotiable advantages of any Exaltation. (This is assuming that the universal Exaltation will be roughly Terrestrial in power - there doesn't appear to be a rung below this, and generating several billion Celestial Exaltations seems likely to be prohibitively difficult.):
Looking at just that basic framework, I would point out that an entire society that has access just to that stuff is pretty well-off. It's not that wide, but it is pretty deep, and considering how much better Thaumaturgy is when you have access to a generous Essence Pool, there are plenty of military, industrial and social applications available. It obviously would not be desirable to have everyone in Creation be able to summon and bind First Circle Demons, but the theoretical possibility of it shows the power-level we're talking about here. So the Silent Exaltation is just a blank slate. It would come with no Charms or specific Essence-tricks of its own, just basic access to the stuff that all other Essence-Wielders get already. It's obviously not as good as any given Terrestrial Exaltation, but its still highly useful, potentially powerful in some focused ways, and massively beneficial to the quality of life in Creation.
This is basically just a variant of the option above, but the Exaltation comes with one specific benefit: a variation on the Eclipse Case ability to learn other Essence-User's tricks. If the power were actually as wide as all that, it would probably be too good, making Mirror Exalts largely superior to any other Terrestrial Exaltation. My thought is to limit the scope to non-Charm methods of Essence-Channeling (so Mirrors could learn powers from the Aalorai and the Dragon Kings, but not from other Exalts). I mostly mention this possibility because I think it would be funny to have millions of humans running around using the Paths of Pre-Human Mastery.
We know that there are five pre-existing types of Terrestrial Exaltation (there may also be a secret sixth hiding somewhere in Atomnos). This is the option where we just inflate the number of Dragonblooded, Star-Crossed, Golden Ones, Beast-Borne and Shadow-Kissed to the point that everyone is covered. This plan has the advantage of a wide variety of useful Exalt types, a certain elegance for dodging the question of just what the universal Exaltation should be, and, of course, nostalgia. It has the disadvantage that it would probably have to work in a completely different way than building something new from scratch, and we don't actually know a lot about how Terrestrial Exaltations work (since we've only ever had Dragonblooded to study). Still, even if we pick a different option, we will probably want to pursue some-sort of restoration of the missing or diminished Terrestrial Types anyway.
One potential resource for the raw power and spiritual material needed for mass Exaltation are Unshaped Raksha. Using them doesn't necessarily mean following this option - we could try to pound out most or all their flavor for the Silent or Mirrored Exaltations, or bend them into specific parameters for some other new type (I'm having trouble coming up with examples of design space not already covered). But we could also embrace the nature of the material, and create the Exalted of Dreams. Most Fair Folk charms don't apply to Creation, and some of the ones that do aren't very good. But there are a few gems (Imposition of Law, I'm looking at you), and the limited ability to dream oneself into a more ideal state (possibly buying mutations without the hefty Essence-disability) has a lot of applications. Appropriating Fair Folk design elements also provides a nice variety of subcategories (sixteen different Castes!).
We've also discussed using the universal Exaltation to promote human unity, which I think this course could serve well. Say, if all Shinmaics share the same dreamscape when they sleep, presenting opportunities for cross-Creation intermingling and promoting mutual understanding. (Here's a thought: what if they can only learn charms and spend experience while dreaming?) One possible drawback to this plan is that infusing the majority of the human population of Creation with the fluid qualities of the deepest Wyld might somehow disrupt Creation's stability This strikes me as sort of a "this atomic weapon test might ignite the atmosphere" problem (ie, one we are collectively reckless enough to ignore) but we should at least consider it.
What we are considering is a fundamental change to the human experience, the very nature of what it means to be human, in fact. That raises some issues; here are three:
Assuming that the universal Exaltation contains some means of promoting empathy and intra-species understanding, what about all the other Exalts that won't share in it? Even if no specific psycho-social component is included, there will still be unique experiences shared by all within the common Exaltation but not with anyone outside of it. This has the potential to further isolate and alienate the "traditional" Exalt types. Exalting all humans will mean that they will no longer be so clearly above the mass of humanity, but they may come to feel more acutely outside it.
The question "what makes a human" is far from academic in Creation. In the case of Wyld mutations, there's a convenient line: once you're so mutated that you can't survive indeffinitely in Creation, you're out of the club. But I can think of at least two other corner cases where the distinction is blurry, and we may not like where it will fall by default:
There are mortal humans and god-bloods all over the planes. I suspect that there isn't any hidden connection between the water system of Meru and everywhere else. Are we tryin to Exalt all humans, or just all humans on Meru? Can whatever method we devise be reapplied elsewhere? (Wassiru probably has rain or something like it, but I wouldn't be surprised if Xara or Atomnos didn't.)