Oh, I quite agree that the Brandenburg Gate should be reserved for historic speeches, and that this one will almost certainly not qualify.

And only those few politicians -- willing to affirm that they, too, are jelly donuts -- should be allowed. /snark

My opinion on whether he gives a speech in Germany or not depends on how many people will show up for it.  If it's going to be many, I say he should go for it, because it'd be helpful for people abroad to make their feelings known (so long as they're on our side, of course).  If not many, skip it.

I don't know how the German public feels well enough to gauge it.  I've read, from people in Germany and those outside of it with some knowledge, that it'd draw a pretty big crowd, but I'm also left thinking, "Surely our election can't be that interesting to people over there."

Be nice to America. Or we'll bring democracy to your country.