I'd say horns are one of the instruments you rarely hear in most music styles nowadays. Horns were very common in music from the 60s to mid 80s. Horn sections added a dramatic sound and feel to a song. Some Brass Construction:
the Hurdy-Gurdy ... other than on the occasional Led Zeppelin tune or medieval dirge. Seriously, if you don't know what these are, google it. It's a fascinating instrument.
I think the vibraphone is under used. I like hearing it used in Jazz. Congas could be used more in various dance music.
I think accordions are pretty neat. I guess they really fell out of style and became a symbol for uncoolness and then had a sort of hipster resurgence (which i guess solidifies and emphasizes them being uncool in some peoples eyes) i was reading about them on wikipedia and there's actually a shit load of different kinds of accordions which is really fascinating, and there are electric accordions that started getting made around the 2010s i think. i guess im not one to complain and feel anything needs more of this or that but i guess it would be cool if more songs had accordion and more variations of different instruments as well. i think its super awesome that the whole rock trifecta of like bass , lead guitar, and drum kits became so important to pop music as a whole but i think we also forget theres such a really rich interesting and beautiful history of the development of all kinds of instruments along with just recognizing different sounds. maybe the way new pop music uses synthesizers so much that some people feel like the categorizations of instruments dont matter anymore dont realize that they are actually bringing a sound/music awareness into the mainstream that used to be represented by the myriad of instruments that exist. this probably sounds pretentious not gonna lie, since entire orchestras are still a thing etc. but you kno what i mean ..
The Sitar was used in early music as an interesting alternative by the Beatles and Steely Dan to name two. I think it should be incorporated into more music backgrounds.