I'm assuming this is different from text/x-web-markdown right?
Reading the Wikipedia entry, would "Markdown Extra" be closer to wiki style text markdown than the current default included list?
Is there a reason it is disable? Security? Stability? Other?
@espenmn how is that enabled?
I see that I do have the file markdown_to_html.py in my /opt/plone/buildout-cache/eggs/Products.PortalTransforms-2.2.1-py2.7.egg/Products/PortalTransforms/transforms
Frankly, I dont know much about 'wiki', but seeing what markdown extra has (which can easily be enabled in Plone), it would fit every need I 'ever need' if it was possible to enable 'add link' (which should be possible by making a a browser view to add new content.
I think (probably) everything you see at https://stackedit.io will work in plone. There is also a wiki link extension that I have not tried: https://pythonhosted.org/Markdown/extensions/wikilinks.html
simply to register the solution here; @vangheem released the following new package:
in which the wiki feature was completely refactored in a clever and maintainable way using portal transforms.
as @tkimnguyen stated: simple and elegant.
congratulations!
@hvelarde lol thx ... @merpdotcom and I were discussing how to crowdfund this but he and his client went ahead and funded the SimpleWiki work 
The linking by ID works fine so far. The title matching linking is fiddlier.
yes, as soon as your content grows or you add different levels you'll start seeing more issues.
a Wiki is a completely different beast and, in Plone, neither ID nor titles are unique; you have to use the whole path or the UUID, but that's not user-friendly.
so, collective.simplewiki could work fine in small, flat sites, and could not scale very well; only time and use will tell.
anyway, is so simple that making it better will be easy.
If someone wants to take a crack at fixing the linking-by-title issues (matching by title is harder) that would be wonderful.
I can imagine one nice enhancement being the display of matching content as you type the ID (or title) so you can pick one.
I'm not sure it's a great way to link. Doesn't mediawiki only allow linking by ID too?
And yes, hector is right about UID... but id is what we have to work with and it isn't too bad for a simple wiki solution. It is called "simple wiki" 
The basic wiki in Pone 4.3, works just fine by Title [[Title Name Here]] to find it. Is there something in 5 that makes this more difficult? Or did the older wiki software have a lot more coded to make it more user friendly?
We've had to reevaluate using 5.5 just isn't there yet, and the integration of so many best of breed products isn't workable. So we're trying to work it from the 4.3 base, with far more options in one platform. But, hopefully the efforts started with SimpleWiki for 5, will be built upon and improved, so that when 5 is ready as a migration path from 4, the remaining wiki features will be worked out by then.
Many thanks & cheers!
Just a note, there seems to be an issue with collective.simplewiki and mosaic