I'm asking this question for a conlang. I know that's normally too subjective for this forum, but honestly, I think this may be an exception.
I tend to try to keep things as terse as possible. Often times my case endings are just the 5 or so vowels of the language. Nothing more. But I was wondering, would this be too minimalist for common use? Not that I intend my conlang to actually be used, but it would just seem weird to me for the fictional people who speak my language to use something that would be unusable in the real world.
You can't have things be too minimalistic, you need redundancy. Humans don't catch every phoneme pronounced, and this problem is even worse over a phone. So in general its a bad idea to have distinctions that differ only in one phoneme. Not that this doesn't happen (English's plural and possessive endings are an example, also there's only one phoneme of difference between 'can' and 'can't'). But looking at languages that do have case systems, they don't just change the final vowel and leave it at that. They often add consonants, sometimes entire syllables, when declining their nouns. Also, I've noticed that languages seem to dislike having endings that are just a single vowel be anywhere close to eachother. For example, in Latin, there is no declination where two of the cases are marked with -i and -e. And in Finnish the only cases that end in a vowel are -a and -i, all the others are differentiated by consonants too. Russian does allow nouns to take an -i or -e ending for different cases, even in the same declination, though in Russian an -e always palatalizes the previous consonant, creating a redundancy.
But looking at Latin and Finnish, I have noticed something. They do allow cases to be differentiated by just a consonant. For example, the Latin first declination singular ending for the nominative is -a, and -am for the accusative. And looking at the second declination again, I see that there is a long -i ending that contrasts with a short -e ending, in the second declination. Still, the different lengths is a redundancy.
I find this interesting that I can't seem to find a language with cases differentiated with a single vowel (or close ones at least), but languages seem to have nothing against cases being differentiated by a single consonant.
Is this universal? If there was a language where the case endings were just -a, -e, -i, -o, and -u, would speakers find these too similar to each other? Would they require some extra redundancy, like a consonant being added to one of the front vowel cases to differentiate it from the other, lengthening one, or just letting the two cases merge into one?
Essentially, this is a question as to how much redundancy is necessary. Obviously, redundancy is necessary, but how little can you get away? What is the minimum redundancy that is necessary? Does anyone even know the answer to this?