Nominal

Inflection: Historical problems and proposed solutions
The concept of Germanic 'Auslautgesetze', or "ending laws", is an acknowledged problem within Germanic linguistics: the daughter languages agree on some endings but widely disagree on others, while some sounds are simply not where or what they are expected to be. There are at least two proposed solutions: the quantitative and qualitative theories.

Quantitative
This theory proposes to solve the endings discrepancy by establishing the differences in vowel quantity, that is, by the inter-Germanic development of trimoraic or overlong vowels: where a normal vowel is transcribed as /ɑ/, a long vowel as /ɑː/, then an overlong vowel /ɑːː/. This theory would account for the difference between a-stem and ō-stem nominative plurals as *-ōz and *-ôz (circumflex signifying overlong), where the latter would theoretically continue earlier pre-Germanic *-eh₂-es (> *-ā-as > *-âs > *-ôz). This overlength of vowels would help explain the long vowels *-ē and *-ō (as in adverbs) which preserve their length even in to the daughter languages where we would expect they would have shortened otherwise, as almost all other word-final long vowels did.

This theory is not widely-accepted among Germanicists, but it was espoused by Don Ringe in "A History of English Volume 1: From Proto-Indo-European to Proto-Germanic", which served as the main source for Proto-Germanic entries on Wiktionary, perceptually skewing the debate quite heavily towards this theory.

Reconstructed/"historical"
The reconstructed forms: forms that are indirectly attested or inferable from the daughter languages.

Regularized/innovative/restorative
With the intention of clarity, distinction and morphological enrichment, this section offers to take the Germanic tradition of regularization to its utmost extent. Changes will be discussed after their appearance or where particularly relevant.

The intended goal of regularizing these nominal declensions is to develop as many unique individual endings ("marking") as can be done while also maintaining the "spirit" of Proto-Germanic and Proto-Indo-European-ness as possible. No additional case forms would need to be (re-)added and/or innovated, eg. allative -at, locative -an, ablative -af (from the addition of prepositional forms, although that would be interesting).

The a- and ō-stems

 * Changes:
 * Masculine vocative *-Ø > -a by analogy with i-stem nom. *-iz, voc. *-i. Alternatively, nom. -az, voc. -az based on the feminine/neuter.
 * Masculine and neuter instances of *-ō- > -ā- to match paradigmatic vowel qualities and distinguish feminine paradigm
 * Masculine and neuter genitive plural *-ą > -ą̄ to match length of feminine and dissociate from accusative singular
 * Feminine genitive singular -ōs introduced by analogy with masc. & neut. to distinguish against plural -ōz
 * Occurrence of unique forms for each gender/stem combination:
 * Masc. a-stem: 11/12
 * Fem. ō-stem: 9/12
 * Neut. a-stem: 7/12

In addition, while Proto-Germanic proper had lost the dual number of declension in nouns, save for a few relics, a dual is still able to be reconstructed as a projection from PIE "backwards", called 'transposition', from comparison with other related languages (Proto-Celtic, Ancient Greek, Sanskrit) or simply by applying the declension paradigm of the numeral "two" (twai(m.), twōz (f.), tū/twō (n.)) and "both" (bai), albeit with some modifications for efficiency. Or even a combination of all the above.

The i- and ī-stems

 * Changes:
 * Masculine, feminine and neuter i-stem singular genitive -is introduced by analogy with a-stems to distinguish from masculine & feminine plurals.
 * Masculine, feminine and neuter i-stem singular instrumental *-ī > -jē to distinguish instrumental from dative.
 * Masculine, feminine and neuter alternate i-stem plural genitive -į̄ proposed to match a- and ō-stem genitive plurals
 * Neuter i-stem alternate nom., voc. & acc. plural -(i)jā proposed to distinguish from dative singular, but not strictly necessary.
 * Occurrence of unique forms for each gender/stem combination:
 * Masc. & fem. i-stem: 11/12
 * Neut. i-stem: 7/12; 8/12 with new plurals
 * Fem. į̄-stem: 9/12

The u-stems

 * Changes:
 * Masculine, feminine & neuter alternate singular genitive -us proposed to match a- and i-stem genitives
 * Masculine & feminine alternative plural nominative and accusative -ūz proposed to match a- and i-stem plurals
 * Masculine, feminine and neuter alternate u-stem plural genitive -ų̄ proposed to match a- and ō-stem genitive plurals

an/ōn-stems

 * Masculine singular nominative *-ō changed to -ā- to match color of the paradigm, as done in (pre-)Old English -a and (pre-)Gothic -a, both from *-ā.
 * Introduction and reintroduction of stem final *-n to better match the paradigm.
 * Leveling of -in- to -an- in the masculine and neuter to regularize the pattern.
 * Unetymological neuter strong case -ō replaced with the cross-linguistically more regularly pattern of -a(n).

in/un/īn-stems
A merging of PIE hysterokinetic *-ḗn, neuter *-n̥ (more commonly appearing in *-mn̥) and the Germanic innovation -ih₂-n- into a single paradigmatic gender triad.