monitor files (à la tail -f) in an entire directory (even new ones)

You can tail multiple files with… multitail.

multitail -Q 1 'directory/*'

-Q 1 PATTERN means to check for new content in existing or new files matching PATTERN every 1 second. Lines from all files are shown in the same window, use -q instead of -Q to have separate windows.

xtail is also an alternative. Its man page describes it as:

Xtail monitors one or more files, and displays all data written to a file since command invocation. It is very useful for monitoring multiple logfiles simultaneously. If an entry given on the command line is a directory, all files in that directory will be monitored, including those created after the xtail invocation. If an entry given on the command line doesn’t exist, xtail will watch for it and monitor it once created. When switching files in the display, a banner showing the pathname of the file is printed.

An interrupt character (usually CTRL/C or DEL) will display a list of the most recently modified files being watched. Send a quit signal (usually CTRL/backslash) to stop xtail.

No idea about a shell solution, but (assuming Linux¹) inotify could be the way to go... see this example imitating tail -F (using pyinotify), maybe it can be used as a basis for following an entire directory.

In general, inotify can monitor directories (citing man 7 inotify)

The following bits can be specified in mask when calling inotify_add_watch(2) and may be returned in the mask field returned by read(2):

IN_ACCESS         File was accessed (read) (*).
IN_ATTRIB         Metadata changed, e.g., permissions, timestamps,
                    extended attributes, link count (since Linux 2.6.25),
                    UID, GID, etc. (*).
IN_CLOSE_WRITE    File opened for writing was closed (*).
IN_CLOSE_NOWRITE  File not opened for writing was closed (*).
IN_CREATE         File/directory created in watched directory (*).
IN_DELETE         File/directory deleted from watched directory (*).
IN_DELETE_SELF    Watched file/directory was itself deleted.
IN_MODIFY         File was modified (*).
IN_MOVE_SELF      Watched file/directory was itself moved.
IN_MOVED_FROM     File moved out of watched directory (*).
IN_MOVED_TO       File moved into watched directory (*).
IN_OPEN           File was opened (*).

When monitoring a directory, the events marked with an asterisk (*) above can occur for files in the directory, in which case the name field in the returned inotify_event structure identifies the name of the file within the directory.

(...and pyinotify closely follows theses options)

1: BSDs have a similar thing, kqueue. Maybe a cross-platform solution is achievable using GIO (Python bindings) as abstraction layer since it can, beside inotify, also use kqueue