Thank you all for your great comments.

@neuro: you are rigth. A TS-E is missing. But it is nothing I am missing for my use cases at the moment.

The cases HDNitehawk is referring to in his 300mm description comes pretty close to what I think I could benefit from an upgrade. You have to know that next year two sisters in law and my borther in law are going to marry. So anything what helps me there is welcome. Of course the 70-200 will do a good job here but knowing the locations already some fast glass with a 300mm focal length could help to get close enough (around 800 guests are expected for at least two of the three weddings). I also like to keep some distance to the subjects so they are not getting distracted.
And sports is also always a topic: basketball and football.

But Rich you are right: The 200-400 is also a lense which caught my attention. But with f/4 I am not sure it is fast enough for all scenarios I am looking for. And I really like fast glasses and the bokeh.
And the image quality? Will it be comparable to the primes? What does the rumors say Rich?
And maybe my wife grants me another 'wish' next year and by than the beauty might have hit the streets already :-)

So rethinking all your comments I still think the 300mm is the best next step for me.