After receiving some feedback and doing some thinking, I am considering changing how my nouns form their plurals, mainly in non-nominative cases.  I am going to use the masculine noun deru, man, to illustrate the current declension of nouns that end in a vowel.

I may nix the -um and place the case marker after a plural marker -n. =>

With my current phonotactics, the genitive, dative, instrumental, allative and locative plurals would end in illegal consonant clusters.  This could be solved with an epenthetic –ï [I] or by reduplicating the last vowel:

The second option would keep all plurals from appearing identical, at least in their surface form, although the underlying form would be -n(case ending)R where R is a reduplicated vowel.  Whichever manner I choose, I would need to decide to the leave the other forms ending in a consonant or if I would add the same vowel by analogy:

Even with this change, this wouldn't preclude the inclusion of other irregularities, like nouns that have some other plural marker, or nouns that use a different stem altogether in the the plural forms.  Decisions, decisions…