It will still happen, but I've updated my priorities so it's been pushed further down the list. This is not a bad thing though, it actually means that the more exciting bits can happen sooner.
The plan WAS that QuestKit would be the next version of Quest. So there would be two systems:
Quest 5.x - a primarily Windows-based system, with a web-based runner and editor (using lots of server-side logic, so they're slower than the offline Windows version).
QuestKit 6.0 - a new JavaScript-based system, that exports games to HTML and JavaScript so they can be run everywhere with no server-side component.
The new plan still starts and ends at the same place, but it has intermediate steps.
Quest 5.x - as before.
Quest 6.0 - the runner will be HTML and JavaScript. This means all Quest games, including existing games for Quest versions 1 to 5, run without a server-side component. They're interpreted by the web browser, making them much quicker. The runner will also be packaged into a cross-platform app, so you'll be able to play all Quest games on Windows, Mac and Linux.
Quest 7.0 - the editor will be HTML and JavaScript. This means the web version of the editor will be the same as the desktop version, with no server-side component. This makes it faster, with no missing features, and it will work on Windows, Mac and Linux.
QuestKit 8.0 can then use an updated version of the editor from Quest 7.0. As previously planned, it won't have its own language at all and will be entirely based on JavaScript. There won't be a runner, as the editor will generate HTML and JavaScript files.
So far I have Quest 6.0 working well with files for Quest 4.x and earlier. I've converted the old VB.NET code into TypeScript which was quite fun. Next step is a new JavaScript interpreter for ASLX files for Quest 5.x, which is progressing well but there's still a lot of work left to do.
Here's Quest 6.0 on a Mac running The Mansion II, a game written for Quest 3.5 and published in 2006...
Depending on how much spare time I get, I hope to have the remaining ASLX support done in the next few months. That should hopefully mean Quest 6.0 will be released some time in 2016. Then I'll start on Quest 7.0 which will take a while, and then finally I'll get back to QuestKit.
The plan for the future of Quest has changed a few times, as you'll see if you browse older forum posts, but I've actually written some code now so this feels like a much more concrete plan than before. It delivers a lot of the advantages of QuestKit sooner - faster online games, the ability to package a game as an app using the same runner etc. Unfortunately it means QuestKit itself is delayed, but I hope that's an acceptable trade-off!