デフォルトの :mkdir コマンドの改良版。
mkdir した後に、そのディレクトリを選択します。

作者からコメントをもらったので更新

commands-mkseldir2.py

from ranger.api.commands import *
class mkseldir(Command):
  """
  :mkseldir <dirname>

  Creates a directory with the name <dirname>, and select it.
  """

  def execute(self):
    from os.path import join, expanduser, lexists
    dirname = join(self.fm.thisdir.path, expanduser(self.rest(1)))
    if not lexists(dirname):
      self.fm.mkdir(dirname)
      if lexists(dirname):
        self.fm.thisdir.load_content(schedule=False)
        self.fm.select_file(dirname)
      else:
        self.fm.notify("Failed to mkdir!", bad=True)
    else:
      self.fm.notify("file/directory exists!", bad=True)

以下は古いバージョン

commands.py

from ranger.api.commands import *

class mkseldir(Command):
  """
  :mkseldir <dirname>

  Creates a directory with the name <dirname>, and select it.
  """

  def execute(self):
    from os.path import join, expanduser, lexists
    dirname = join(self.fm.thisdir.path, expanduser(self.rest(1)))
    if not lexists(dirname):
      self.fm.mkdir(dirname)
      if lexists(dirname):
        item = ranger.fsobject.directory.Directory(dirname, path_is_abs=True)
        item.load()
        self.fm.thisdir.files.append(item)
        self.fm.select_file(dirname)
      else:
        self.fm.notify("Failed to mkdir!", bad=True)
    else:
      self.fm.notify("file/directory exists!", bad=True)

Veedrac さんに教えてもらいました!!!
https://bbs.archlinux.org/viewtopic.php?pid=1155529#p1155529