@Hanna_Petruschat_WMDE Curious in fully understanding your question: You're asking how to best possibly proceed in transforming an adult product over to make use of the WikimediaUI Style Guide conventions, correct?
What do you mean by “implementation plan for desktop”?

Yes, that's what I meant with implementation for desktop. As of now, I just see the styleguide being applicable for the mobile version of Wikipedia, while we (the UX team at WMDE) are focussing on adjusting things for the desktop version. So I was wondering if there is a plan to slowly adopt the desktop layout to the mobile style guide or if there will be a hard cut, shifting the whole desktop layout to the mobile one.

I'm basically asking for a framework on how to handle layouts for desktop features. Let me give you an example:
Maybe you heard about the Edit conlict and that we are currently working on that with the Technical Wishes Project. As of now, we have a simulation running as a beta version which from a layout perspective fits in the classical desktop style.
This offers two ways to go:

1. As this task includes the design of several pages I could see the opportunity to adjust the layout to the current style guide. This would mean a visual break for the users, but could be a chance for a step-by-step-shift.
2. And if we don't use the current style guide: What do I take as a guideline instead? I could look at other pages and try to make it work, but I feel this would just add to the existing clutter.

I hope this brings a bit of clarification to what I just briefly described above. Feel free to ask further questions and/or share your thoughts on that.