And we have a winner, that's a pretty good build.
There is no such thing as future proofing though. The most cost effective way is to buy exactly as much power and features as you need.
Also you didn't specify a budget.
There's not much you could do besides some cosmetic changes. Most of them are literally cosmetic.
RAM: Instead of 1600Mhz CL8 in blue you could get 1600Mhz CL9 (slightly slower) in blue (-2$) or red (-6$) or 1866MHz CL9 (slightly faster) in red (-5$) or 2133MHz CL10 (a bit faster again) in gold (+1$).
You could go microATX (-66.68$).
microATX complete blue colour scheme (minimal performance sacrifice) for 12$ less.
Taking that build and swapping the i5-4690K for a Xeon E3-1231 v3 costs 15.11$ more. Not overclocked the i5 is only 3% faster in single threaded applications and even overclocked it can't beat the Xeon in multi threaded. So in other words, single threaded the i5 beats it barely and is cheaper, overclocked it beats it convincingly but at a higher price. Multi-threaded the Xeon wins against non-OC by a wide margin while being only slightly more expensive and while the gap is smaller against OC it would still win and be cheaper.
The original GPU (+6.01$) and red colour scheme would add 12.92$ to the Xeon build.
Short summary:
Original: 846.50$
microATX: 779.82$
mATX blue: 767.82$ (slightly slower RAM and GPU)
Xeon mATX blue: 782.93$ (slightly slower RAM and GPU)
Xeon mATX red: 795.85$ (slightly slower RAM)
And now my personal favourite.
Only works if you're willing the go through flashing-the-BIOS-without-a-CPU process.
Same performance except for the RAM and it's the second cheapest build:
PCPartPicker part list / Price breakdown by merchant
CPU: Intel Core i5-4690K 3.5GHz Quad-Core Processor ($219.88 @ OutletPC)
Motherboard: MSI Z87M GAMING Micro ATX LGA1150 Motherboard ($79.99 @ Newegg)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 Memory ($63.99 @ Newegg)
Video Card: MSI GeForce GTX 970 4GB Twin Frozr V Video Card ($347.99 @ Amazon)
Case: Cooler Master N200 MicroATX Mid Tower Case ($39.99 @ Newegg)
Power Supply: Corsair CSM 450W 80+ Gold Certified Semi-Modular ATX Power Supply ($26.99 @ Newegg)
Total: $778.83
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-01-17 07:39 EST-0500