Hi,
When this topic was raised before a few years ago (I dont remember which time, it's been continuingly discussed throughout the years) I found an idea especially interesting but it got buried in the mass.
From memory and imagination:
The idea is to write a new parser that is not deep in MediaWiki and can therefor be used apart from MediaWiki and is fairly easy to be translated to, for example, javascript.
This parser accepts similar input as we do now (ie. '''bold''', {{template}}, [[link|text]] etc.) however totally rewritten and with more logical behavour. Call it a 2.0 parser without any worries about compatibilty or old wikitext edge cases which (ab)use the edge cases of the current parser.
This would become the default in MediaWiki for new pages created, and indicated by an int in the revision table (ie. rev_pv (parserversion) ). A WYSIWYG editor can be written for this in javascript and it's great.
So what about articles with the old paser (ie. rev_pv=NULL / rev_pv=1) ? No problem, the old parser stick around for a while and such articles simply dont have a WYSIWYG editor.
Editing articles with the old parser will show a small notice on top (like the one for pages larger than x bytes due to old browser limits) showing an option 'switch' it. That would result in previewing the page's wikitext with the new parser. The user can then make adjuistment as needed to make it look good again (if neccecary at all) and save page (which saves the new revision with rev_pv=2, like it would do for new articles).
Since there are lots of articles which likely will have the same output in HTML and require no modification whatshowever there could be a script written (either as a userbot for the end user or as a maintenance script) that would automatically check all pages that have the old rev_pv and compare them to the output of the new parser and automatically update the rev_rv field if it matches. All others would be visible on a SpecialPage for "pages of which the last revision has an older version of the parser", with a link to an MW.org page with an overview of a few things that regulars may want to know (ie. the most common differences).
Just an idea :) -- Krinkle