Hi! I finally have access her again! (you can all thank zas! )
Like mfmeulenbelt said, the JIRA ticket system is where to ask for new instruments or improvements on old ones.
Now for the question here: as has been discussed, it seems the “contrabass guitar” is something mostly by one artist, as such it’s usually (usually) not valid.
It also seems that “contrabass guitar” could mean several things; this makes it difficult to figure what use is meant in liner notes, and which (if at all any should actually be added to MusicBrainz)
Finally not all parts of the instrument-tree have been fixed, updated or completed, (yet) currently I am working on https://tickets.metabrainz.org/browse/INST-392 and https://tickets.metabrainz.org/browse/INST-964 (I’m also working on BookBrainz relationshipstyle, but I digress)
What helps is thorough and comprehensive tickets - even something that has been rejected before can, if explained better, be accepted
For the contrabass guitar I will probably revisit this once I do the “guitar-family” mini-version - who knows? Maybe my research will mean that we do get an instrument called this, it all depends on use and reality :D