I have an idea on how Octoling grammar could be. Feel free to implement or reject any part of it.
Syllable structure:
CVEG
C: any consonant. (opt)
V: vowel. [i e ë a å o u ü]
E: syllable ending. [Vowel lengthener i u n] (opt)
G: gemination of following syllable's leading consonant. If next syllable has no leading consonant, will be rendered as glottal stop. (opt)
I'm also thinking of allowing some consonant custers like [kp][sšþlr], [sšþ][krpt], or [pbfþð]r.
Accents: penultimate. Exceptions may apply for lengthened vowels and should be indicated with acute accent when transcribing.
Main sentence structure: SV, SOV, SDIV (Japanese / German hybrid). OSV, SVO, or VSO are possible by using particles.
Example sentence structure:
[Subject] [Subject particle] [Object] [Object particle (optional/implied)] [Verb] [Question indicator (optional)]
Parts of speech
Nouns - word ending: anything other than [-u], [-na], or [-ni]; gerunds take form [-on]
Verbs - word ending: infinitive takes [-u]; conjugates by case but not person or number
Adjectives - word ending: [-na]; usually before the noun described.
Adverbs - word ending: [-ni]; usually before the verb or adjective described.
Articles - not used.
Particles - subject indicator, object indicator, predicate indicator/separator used when changing word order, question indicator. Question indicator may be rendered unnecessary by conjugation in full sentences (interrogative form [-ay]).
Miscellaneous
Infixes inserted before verb ending
[-as], [-usas], [-usasas], [-ususasas]: causation. First is direct causation, second is indirect 2-level causation, third is 3-level causation or deeper. fourth is 4-level but is optional.
[-er]: passive tense.
[-eb]: repeated or constant action.
[-ab]: verb intensifier.
[-ün]: conditional mood (as in "wäre, hätte, spräche")
[-nčën]: added after noun for diminuitive. Roughly German -chen, Spanish -ito/-ita, Japanese chan/kun
Borrowings should be from Japanese or German, or preferrably cognates with Inkling. e.g. "tentacle" can be [nežuzo] (<Inkling), [šokušü] (<Japanese) or [fangaamë] (<German), preferred in that order. Be sure to occasionally mutate with special non-Inkling/Japanese/German sounds when borrowing.