In my conlang, things are pluralised by lengthening the first vowel and softening the first consonant in some way, usually voicing, e.g. þoli > ðóli (person), henafa > énafa (colour), etc. (Pronounced roughly [ˈθɔli ˈðɔːli ˈhɛnɐfə ˈæːnɐfə], by the way.)
Leaving aside that this is a slightly silly way of doing things, it seems far too regular to apply to all nouns. And the language is mostly agglutinative so perhaps the plural should also be an actual affix rather than a weird mutation thing.
So, my question is: how plausible would you find the above plural in an agglutinating language (or at all)? I'm thinking of chucking it out and replacing it with something more fitting, but it might take a while to mentally readjust to the new one. :p This is also one of only two places where mutation features in anything; I haven't got around to doing any more because it would mess up the plurals.