Let's assume for a moment BMW actually does make the new 6-Series lighter and more focused on the driving experience. This would then leave more room for an 8-Series variant to slot in between the upcoming 7 and 9-Series. BMW makes the 6-Series less of a GT and more of a sports car meaning the 8-Series will be the top GT in the lineup. It actually makes sense as long as the 6-Series doesn't get too close to the 4-Series.
The next 6 is a ways off. A lot could change and AutoBild has no cited sources for their report. It just feels like they are guessing, like most publications looking for hits above all else these days.