(Originally posted on twitter, here incorporating responses)
As of version 2.6, there is provisional support for using pandoc using jupyter notebookss (.ipynb files) as input and output format.
Markdown to Jupyter Notebook, and back https://github.com/aaren/notedown
Restructured text (rst) to Jupyter Notebook https://github.com/QuantEcon/sphinxcontrib-jupyter, see https://medium.com/quantecon-blog/introducing-jupinx-60ba9fc12f4f
Doconce format to Jupyter Notebook and back https://github.com/hplgit/doconce, see http://hplgit.github.io/doconce/doc/pub/manual/manual.html#do2any:ipynb
Python script to Jupyter Notebook http://sphinx-gallery.readthedocs.io/en/latest/index.html, see http://sphinx-gallery.readthedocs.io/en/latest/utils.html?highlight=jupyter#convert-python-scripts-into-jupyter-notebooks
nb2plots converts Jupyter notebooks to ReST files for Sphinx, and back again. https://matthew-brett.github.io/nb2plots/
org-file to Jupyter Notebook https://github.com/jkitchin/ox-ipynb
YAML format to Jupyter notebook and back https://github.com/prabhuramachandran/ipyaml
A bit unclear from the documentation, but seems to be able to generate markdown from Jupyter Notebook, and back? Also other formats? https://github.com/dexy/dexy. Markdown example: https://github.com/ananelson/dexy-experiments/tree/master/ipython-from-markdown. Other exmaples: https://github.com/ananelson/dexy-experiments
Converts between IPython/Jupyter notebooks and R Markdown documents in both directions. https://github.com/chronitis/ipyrmd
With Jupytext, every Julia, Python or R script, R Markdown or Markdown document becomes a potential Jupyter notebook. Write your notebooks as text, and render them in Jupyter when desired. https://github.com/mwouts/jupytext Blog post: https://towardsdatascience.com/introducing-jupytext-9234fdff6c57.
Manage Jupyter notebooks as plain Python code with embedded Markdown text. https://github.com/minodes/pynb.
An Experimental New Storage Format For Jupyter Notebooks. Documentation: https://jupyter-format.readthedocs.io. Repo: https://github.com/mgeier/jupyter-format
A slightly different approach. Explode a notebook into a directory of files, and recombine it to a .ipynb https://github.com/takluyver/nbexplode