Hi, I'm not a fan of buying for tomorrow as it adds too many uncertainties into list of actual requirements.
I use the following lenses on FX and DX; 16-35mm f/4, 24-120mm f/4, 70-200mm f/2.8 plus several primes. For me, the 24-X lenses are at home on FX and a bit awkward on DX, just not wide enough for what I do. OTOH, my wife uses a 24-85mm on DX and finds it very useful.
Some of this choice depends on how frequently you want to change lenses. You already have the 70-200mm f/4 so the 24-120 provides a lot of overlap. What's curious in all this for me is I find the 24-120 so useful that I frequently forego my 70-200. In a really backward argument you could use this to recommend the 24-70. That's the problem with not having usage driven requirements. If you try to drive your selection on strictly coverage and maybe-in-the-future you can miss something important in today's needs.
I find the f/4 lenses to be quite useful. You do loose some DOF creativity on DX, but even with the f/2.8 lens on DX there not a great difference at the wide to mid-range focal lengths so IMO there's not a lot of f/2.8 or f/4 differentiation that drives this selection. Things change a bit on FX, but many are ready to forego the f/2.8 benefit on FX for the increased versatility of the f/4 and the lighter weight.
This all sounds like a hodgepodge, but I hope it spurs you to articulate your requirements a bit better.
If it's really just about sharpness, you still have to balance the inherent optical qualities of the f/2.8 against the terrific VR benefit of the f/4. There are situations where each dominate. I prefer the VR.
Roger It's still, ISO, aperture and shutter-speed, right?