I think the term is pretty much established:
https://www.emacswiki.org/emacs/FindOtherFile
Projectile also has a command with a similar name.
Then the name 'other-file' is fine.
The feature will be pretty C/C++-centric, but I suppose it's useful enough.
It's also useful with Web development: while VueJS can keep parts of
components in one file .vue, other frameworks require 3 different files:
file.html, file.js and file.css.
A fuzzy search across full names works best for me, personally. We could
also try some differently weighted, fuzzy matching styles.
I don't know if the required completion can be part of a fuzzy search,
but it should show exactly the same completions as the current command
(with directory and file names), but should match only on file name parts,
not on directory name parts, e.g. with files:
dirname/filename1 dirname/file2
typing "name" for completions should show only "dirname/filename1",
because most of the time, project directory names get in the way
when searching for a file by its name, and part of file name matches
directory names with many other unrelated files.