Interesting thread, I went through all the answers and proposals.
Time now for my idea, I couldn't post earlier because of traveling and I had not my password to access RFF on the phone!
As principle I do not need more gear...but that Rolleiflex 2.8...so beautiful...
I must say that years ago I found more interesting to spend money on workshops than buying gear. This was a good way to meet other people and see what they were doing (or trying to do), get new ideas for new projects and new challenges. The biggest result I got from the WS i took part in between 1995 and 2005 is not a technical knowledge but to know what I want to achieve with my photography. Which style or better which visual language to use in my projects. So this could be an idea, we have an interesting "masterclass" which includes a series of 5/6 weekend with a photographer and later a meeting with an editor, with the aim to learn how to finalized a project into a book.
The total cost (fee + material, printing papers...) is about 2.500/3.000 EUR
Thinking only in terms of gear a Nikon DF seems me the correct complement to my M10 and I already have the lenses I need. But I'm not sure I need more gear, already said, I know!
Because I like printing very much a good inkjet printer (A2 ...) and a stock of inks and high quality paper would be also an interesting idea...
SO, no one of the proposal made by the OP is for me and if I should buy another Leica the TL2 is very appealing...I already have the EVF which is the same of the M10
Thanks Raid for letting me think about what to buy with some money which is not (yet?) in my photo expenses budget