TL;DR: It would be nice it the switcher for the diff modes (visual and wikitext) would be easily extensible, especially for historical diffs.
I have a user script, https://de.wikipedia.org/wiki/Benutzer:Schnark/js/diff, that adds an additional diff mode (similar to WikEdDiff). Currently, it uses it's own switcher (the tabs visible in the screenshot). This causes a bad user experience with the two different switchers, so eventually I want to adapt my script to add to the diff mode switcher provided by VE.
For the diff in the save dialog this is actually quite easy, all I'd have to do is to extend ve.ui.MWSaveDialog.prototype.initialize to add to this.reviewModeButtonSelect, and ve.ui.MWSaveDialog.prototype.updateReviewMode to add my listeners.
But for historical diffs, the situation is much less ideal: Not only is it impossible to reuse any of the code for the save dialog, nor is the switcher there accessible and thus extensible.
Of course, just one user script isn't enough to justify some complex code in VE, but VE is designed in most of its parts to be easily extensible, so I'd be really happy if that applied to ve.init.mw.DiffPage.init.js, too.