Assuming this is about Japanese voicebanks, you may be interested in this post, which discusses standard and non-standard Japanese syllables.
https://utaforum.net/resources/what-to-look-for-in-a-japanese-reclist-and-what-to-avoid.170/
As for my personal preferences, I have written two main CVVC reclists for Japanese + Extras.
One is sunetoha, which strictly obeys Japanese phonotactics while going "overkill" on extras that appear in expressive Japanese pronunciation. There are 4 variants of ん corresponding to the sound assimilation rules prior to other consonants. There are nasal vowels for the earlier discussed situation where ん sounds like it's part of the previous syllable and not a syllable on its own. There are devoiced い and う and consonant releases for all voiceless consonants, for situations like です/ます. There's "ng" samples, which is a variant pronunciation of "g" samples. There's full VCV for all h/hy/f samples. This is all in addition to standard Japanese CVVC features, such as the distinction between palatalized and non-palatalized consonants and [n C] transitions.
The other is Swordheart Offline, which I actually use regularly. Most of the extras in this case are aimed at enhancing English capabilities, which saves me the effort of having to record a separate English voicebank. It doesn't do English as well as a dedicated voicebank would, but it gets me most of the way there and I find it fun to use.
Like sunetoha, it includes standard Japanese CVVC features and 4 variants of ん. It includes one extra vowel (the schwa) paired with every consonant. There are consonant releases for all of the consonants as well as end-breaths. There's a supplemental list for all English CC transitions. There's full VCV for h and hy. The extra, non-Japanese consonants are ng, zh, dz, English F, l, English R, v, voiceless th, and voiced th (written as dh). These and all standard consonants are matched up to every vowel instead of obeying phonotactic limitations, meaning that, for example, there's a full set of tsa tsi tsu tse tso ts@ instead of only having a tsu.