I've made an overview of all options that need to be added and defaults that need to be changed in ls to reach full compatibility with GNU ls. I hope this might help people out who want to contribute, without adding to many issues to the issue list.

The items marked with 1️⃣ are the ones I think make Good First Issues.

• Formats
  • -x & --format=across & --format=horizontal
  • -m & --format=commas
  • Make -n overrideable (similar to -o and -g)
• Date & time
  • --time=birth & --time=creation
  • --time-style=full-iso
  • --time-style=long-iso
  • --time-style=iso
  • --time-style=locale (works, but not locale-specific yet)
  • --time-style=+FORMAT like in date
  • posix- prefix for --time-style
  • --full-time
  • Default: follow environment variable TIME_STYLE or locale
• Escaping and quoting
  • -b. --escape& --quoting-style=escape
  • -Q, --quote-name & --quoting-style=c
  • -N, --literal & --quoting-style=literal
  • --quoting-style=shell & --quoting-style=shell-always
  • --quoting-style=shell-escape & --quoting-style=shell-escape-always
  • --quoting-style=locale
  • --quoting-style=clocale
  • Default: follow environment variable QUOTING_STYLE or shell-escape.
  • Space in front of file names without quotes if there are names with quotes.
• Control characters
  • -q & --hide-control-characters
  • --show-control-characters
  • Set default to -q if the output is a terminal.
• Sorting
  • -U & --sort=none
  • -v & --sort=version (also used in sort, so might make sense to put this in uucore)
  • -X & --sort=extension
  • Make sorting by name locale-specific (currently only C-locale is implemented). Among other problems, this causes a behaviour where hidden files (prefixed with ".") are sorted before other files, instead of the leading "." being ignored.
  • 1️⃣ --group-directories-first
  • -f (enable -aU, disable -ls --color)
• Dereferencing
  • -H & --dereference-command-line
  • --dereference-command-line-symlink-to-dir
• Hiding
  • --hide
  • -I & --ignore
• Indicator style
  • --indicator-style with options: none, slash, file-type, classify
  • -p (equivalent to --indicator-style=slash)
  • --file-type (equivalent to --indicator-style=file-type)
• Block size (these should mainly be implemented in uucore, since they are used by multiple programs)
  • -s & --size
  • -k & --kibibytes (equivalent to --block-size=1K)
  • --block-size
  • Default: Determine block size from environment variables.
• Loose ends: The options that didn't fit into the categories above
  • -w & --width
  • -T & --tabsize
  • -Z & --context
  • -D & --dired
  • --hyperlink
  • Show total size of directories in the long format