I thought hard about this, but there definitively is no way for a Wiki to be any more minimalistic than Ben Nolan’s Gunja. Like TiddlyWikis Gunja lives in a single HTML-file, but forget about all fancy fading effects, all you get is:

a square box for entering text
a button for creating a link from selected text
a button for going to your start page

And this is where the magic happens: that’s basically all you need (for many use cases). Check it out, it’s focused and fun