Perhaps some steel? Or titanium if you want something a little darker. If you enjoy the Lange, why not get one in honey gold? An interesting ceramic watch might do well for something VERY different.
I think a great way to diversify is dial styles, not metal. Platinum is a great watchcase material in my opinion (however I have not owned one, just read about the metal's properties). I think you would do well to get a nice Seiko, an ultra high end one. Like the Credor Eichi II. That would be just amazing. Mind boggling... it would compliment the Swiss and German design language too. With some strap changes, you'd have an amazingly versatile collection.
Your collection is really nice, very tasteful I think. I look forward to seeing more of it! Perhaps some nice movement and wrist shots?