A Phonology Without Phonemic Consonants

There are languages that have been analyzed as lacking phonemic vowels, with all vowels being completely predictable from the consonant string. That doesn't mean that they aren't pronounced with vowels, merely that vowels serve no contrastive function.

So, how about a phonology that does the exact opposite: packs all of the contrast into underlying phonological vowels, with phonetic consonants being completely predictable from the vowel string?

Now, there are lots of ways to do this in an ad-hoc manner. Say, an /i/ and an /a/ always have a [t] inserted between them, while a [u] and an [a] get a [ʒ], just because. But I'm gonna look at something that is potentially naturalistic, where the choice of consonant phones and where they get inserted is a reasonable consequence of vocalic features. There are probably lots of ways to do that, too, but here's just one of them:

For simplicity, we'll start with just /i/, /a/, /u/ as the basic, plain vowels. You could use more, but these are sufficient to demonstrate the rules I have in mind, which I will describe in terms of generic vowel features such that one could add more basic vowels and already know exactly how they would behave. Each of these can come in plain, breathy, rhotic, and nasal varieties, or any combination thereof; i.e., one could have a breathy nasal rhotic vowel, with all three extra qualities at once. I'll assume that any vowel can have any combination of these qualities, and there are no phonotactic restrictions on the underlying vowel string (although certain combinations might require sticking in a syllable boundary to break them up). Changing either of those assumptions could introduce further structural interestingness in other similar phonologies.

All of these vowels can also be long or short, with syllables being maximally 3 morae; thus, one can have one short vowel, two short vowels, three short vowels, one long vowel, or one short vowel and one long vowel per syllable, where all of the vowels in a single syllable must share all of their voicing, rhotic, and nasal features. For consonant-induction purposes a "long syllable" is any syllable containing a long vowel, or a triphthong (long, long+short, short+long, and short+short+short, but not short+short). Ignoring length, this results in 8 possible versions of every basic vowel, which can be transcribed as V, hV, Vn, Vr, hVn, hVr, Vrn,  and hVrn. That results in a total of 24 phonemes:

i /i/ a /a/ u /u/
hi /i̤/ ha /a̤/ hu /ṳ/
in /ĩ/ an /ã/ un /ũ/
ir /i˞/ ar /a˞/ ur /u˞/
hin /ĩ̤/ han /ã̤/ hun /ṳ̃/
hir /i̤˞/ har /a̤˞/ hur /ṳ˞/
irn /ĩ˞/ arn /ã˞/ urn /ũ˞/
hirn /ĩ̤˞/ harn /ã̤˞/ hurn /ṳ̃˞/

Or 48, if we count the long versions as separate phonemes.

Tautosyllabic vowels can turn into glides. An /i/ becomes [j], while short /u/ turns into [w]. In long syllables, medial vowels are glided first, such that, e.g., /uia/ becomes [uja], not [wia]. Sequences of /iii/ become [ji:] and /uuu/ become [wu:]' sequences of /aaa/ must be broken into two syllables, either [a:.a] or [a.a:]. Since all vowels in a syllable must have matching features, we can romanize these by grouping the vowels together within one set of voice/breathy/nasal letters. E.g., huin /ṳ̃͡ĩ̤/ [w̤̃ĩ̤], or iar /i˞͡a˞/ [ja˞].

That provides us with two phonetic consonants so far: /j/ and /w/.

Other consonants are induced when transitioning from a vowel that has a certain quality to one that doesn't, or at syllable or morpheme boundaries.

Breathy-voiced vowels basically induce an onset [h] (hence the romanization convention) morpheme-initially or after a non-breathy vowel, but in certain situations this can be mutated into aspiration or lenition of a previous phonetic consonant instead (see below). (A reasonable phonotactic restriction on the vowel string might be that plain vowels can't follow breathy vowels in the same morpheme, just because I find it difficult to perceive the transition from breathy to plain voice. But I'll ignore that for now.)

Plain vowels induce plain coda stops. High front vowels (/i/) induce [t], high back vowels (/u/) induce [k], and low vowels (/a/) induce [ʔ]. These all become aspirated when followed by unstressed breathy voice, absorbing the [h] induced by the following vowel unless it crosses a morpheme boundary; if the breathy syllable is stressed, then [t] becomes [t͡s] and [k] becomes [x], again replacing the [h] induced by the following vowel unless it crosses a morpheme boundary, while [ʔ] is unaffected.

Now, we have 8 more phonetic consonants: [t], [tʰ], [t͡s], [k], [kʰ], [x], [ʔ] and [h].

Since plain vowels already lack other properties, so there are none to lose, these consonant sounds will not occur every time there is a transition to a different class of vowel. Instead, they will only occur at non-utterance-final morpheme boundaries if the syllable is short, and at any non-word-final syllable boundary if the syllable is long; additionally, the consonants will be geminated in long syllables, stealing duration from the vowel. Thus, something like <uuha> /u:a̤/ or <iiha> would be rendered phonetically as [uk:ʰa̤]/[ukxa̤] or [it:ʰa̤]/[itt͡sa̤] respectively, depending on stress placement, and assuming it's monomorphemic.

Nasal vowels induce nasal stops, with non-back vowels (/i/ and /a/) inducing a coda [m] at the end of a word and [n] in other positions, and back vowels (/u/) inducing /ŋ/. Nasalization also interacts with syllable length; like the induced plain stops, induced [n] and [ŋ] will steal length from a long nucleus and become geminated.

Successive non-breathy nasal vowels in different syllables induce an epenthetic [ʁ]. Why? Because that's what I discovered I naturally do when trying to pronounce them! I don't know what exactly the articulatory phonetic justification, but there must be one! Thus, something like <unan> /ũã/ comes out as [ũʁãm], while monosyllabic long /ĩĩ/ (romanized <iin>) is distinguished from disyllabic /ĩ.ĩ/ (romanized <inin>) by the phonetic realizations [ĩ:n] and [ĩʁĩn], respectively. The results of a genuinely rhotic initial vowel (<irnin> /ĩ˞.ĩ/) look different still, as described below.

So far, that adds another 4 phonetic consonants ([m], [n], [ŋ], and [ʁ]), for a total of 16.

Rhotic vowels get a little complicated, due to interaction with other qualities. With combined rhotic+nasal vowels, the coda consonants are ordered R-N.
High front non-nasal vowels (/i/) induce [ɾ] word-medially, and [ɹ] word finally or with nasalization. Low non-back non-nasal vowels (/a/) induce [ɹ], which is ambisyllabic word-medially unless it is produced by a non-breathy vowel followed by a breathy vowel (with an induced [h] onset pushing the ambisyllabic [ɹ] out of the way). Nasal mid vowels induce [ʐ] or [ʒ] in free variation. Back vowels (/u/) induce [ʀ] or [ʁ] in free variation, which like [ɹ] is ambisyllabic unless followed an induced onset [h] or paired with nasalization.

That's another 3-ish phonetic consonants, leaving us with a total inventory looking something like this:

Stops: [t], [tʰ], [k], [kʰ], [ʔ]
Fricatives/affricates: [h], [x], [ts], [ʐ]/[ʒ]
Nasals: [m], [n], [ŋ]
Rhotics: [ʀ]/[ʁ], [ɹ], [ɾ]
Glides: [w], [j]

which really doesn't look that bad! It's sorta-kinda Iroquoian-looking, if you squint, with extra rhotics. Several natural languages get along with fewer consonant phones than that. But, it can still be written mostly-unambiguously (save for specifying morpheme/syllable boundaries) purely as a string of vowels from a 24-character all-vowel alphabet; or perhaps a featural script with three basic vowels and diacritics for the various combinations of nasal, rhotic, and breathy features, and maybe length.

Of course, there are other possible re-analyses of words generated this way. The romanization scheme already embodies one: a three-vowel, three-consonant analysis, where the consonants and vowels have some fairly complex interactions generating a lot of allophones of each, and some particular strange distributional restrictions (like, /h/ is the only consonant that can start a word!) A native speaker of such a language might, however, go for a four-consonant analyses, adding /t/ → [t], [tʰ], [k], [kʰ], [ʔ], [ts], [x]; or even breaking things down further, with no realization of the significance of the extremely limited distribution of these sounds. Speakers might also group things like /t/ →
[t], [tʰ], [k], [kʰ], [ʔ], [ts]; /h/ → [h], [x]; /z/ → [ʐ], [ʒ]; /r/ → [ʀ], [ʁ], [ɹ], [ɾ]; based on perceptual similarity, thus confusing the disparate origins of [h] vs. [x] and masking the commonality of [ʐ] and [ʒ] with the rhotics.

If one were to start with something like this and then evolve it historically, one could easily get a more "normal"-looking inventory (e.g., maybe that tap [ɾ] ends up turning into an [l], and maybe [t͡s] simplifies to plain [s]) with a steadily more opaque relationship to the underlying vocalic features, despite still being regularly predictable from them.

If one were to do an intrafictional description of the language, such as might be written by native linguists, I would be somewhat inclined to go with one of these alternative analyses as the standard native conception, and then dive in to the argument for why it should be re-analyzed as consisting purely of underlying vowels instead. Although, it would be a shame to miss out on the opportunity for a native writing system consisting of a 24-vowel alphabet.