e.g. Anthony Jackson is a well-known contrabass guitar player.
(I hope somebody from MusicBrainz can pick this up from here. I am a bit confused if this should be reported/discussed here in the forum, in a irc chatroom, or through a ticketing system, and at the moment I prefer just to stay here 
Could we keep just our current set of bass guitars (electric and acoustic)?
Because it seems just a six string acoustic bass guitar and the name only exists in English.
You can select acoustic bass guitar and use instrument credit contrabass guitar if it’s what the booklet says.
According to your Wikipedia article:
It is popular in Fado bands in Portugal and South America.
And they say it’s really like an acoustic bass guitar except:
Its strings are much thinner than a conventional acoustic bass guitar, so it lacks the “thick” tone of those instruments.
Does that make it eligible for an acoustic bass guitar sub instrument in our tree? Or just instrument credit?
The rule used to be that if you find 5 different releases crediting this instrument, you could make a request. 
There are 2 occurrences in this ticket:
Appears on:
- Hearts and Bones
- Move (played by Anthony Jackson on all tracks, currently credited as bass guitar with a credit)
I think it has to reach 5. 
After some more research, it looks like it’s a bit more complicated than I first thought.
There seem to be a couple of opinions and interpretations of what a contrabass guitar can be.
I checked some guitar builders and a couple of artists, and it looks like it can be:
So it can probably be argued that it is not a specific instrument that is easily defined.
You will probably be able to find more than five releases where Anthony Jackson is credited playing a contrabass guitar, but adding an instrument for mainly one musician might be disputable.
Still, he might deserve it 
So maybe instrument credit is good enough (it is often useful), like done for the 2 ticket linked examples.
@CatQuest @CatCat cannot access this forum at the moment but thinks this instrument belong to Novelty instruments Not to be added as instruments.
See https://musicbrainz.org/doc/How_to_Add_Instruments
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