If the $25,000 were just for camera gear, but I didn't have to buy the gear right away, then I would invest the money and spend it over time on camera gear. Assuming taxes don't apply for some magical reason and it's in USD with no S&H.
If I had to spend it right away:
- D750 as second camera body - $2000
- Nikon 600mm f4e - $12,296
- Second flash (SB-700) & Good tripod for 600mm f4e - $1000
- Nikon 70-200 f2.8 VR II - $2400
- Tamron 24-70 VC - $1250
- Tamron 15-30 VC - $1200
- Nikon 105mm Macro - $769
- Sony A7R + Sony 24-70 f2.8 ZA SSM II - $4000
Reasoning:
Second D750 <-- Just because I can. I could leave one at the girlfriend's house. Plus what if I lose or break my current one??? :O
600mm f4e <-- With the reduced weight, and since it's a new release, should make for a lot of fun wildlife shooting.
Second flash & Good tripod <-- Honestly if someone walked up to me and offered $25k, I would probably consider investing even a little more into this sort of stuff, maybe $2k of the budget rather than $1k
Nikon 70-200 f2.8 vr II <-- Pretty much a staple if you've spent $25,000 on gear
Tamron 24-70 VC <-- Not much to say. I'd rather save over the Nikon-brand 24-70, and there's other things I wanted to fit into the budget.
Tamron 15-30 VC <-- The Nikon 14-24 is better (build and flare), I originally put it down, but then I tried to fit an A7R into the budget.
105mm Macro <-- For macro photography.
Sony A7R + 24-70 f2.8 <-- A lighter, smaller setup to haul around more often? Idk. An alternative is investing the $4k here into maybe a 24mm 1.4 Sigma Art, and lots lighting equipment.