Raw HTML would be nice, but it often leads to security issues.
The immediate second thought would be to add additional text formatting options.
I guess that works "ok" but would probably end up overcomplicated, in my opinion at least.
So I came to this "simple" idea, why not have something similar to the column/row blocks from the My Page layout?
I don't think it's necessary to be able to completely change the layout of the Wiki, but being able to divide individual pages with columns/rows would be extremely helpful when trying to organize them.