i5 - 4690k vs xeon 1231v3 vs fx 8350 future dx 12 choice ?

It would help to know what motherboard brand/model you have. Some boards do fine while others aren't so great at handling a 125w tdp cpu and lack the voltage regulator heatsinks and power phases to truly handle an 8350 well. Not all am3+ boards are the same in that respect and can lead to thermal throttling and less than optimal performance once the fx 8350 is under load.

From what we've seen so far dx12 isn't going to make the fx 8350 take the lead from intel, it doesn't change the cpu's performance. The cpu's struggling most at the moment will be the ones to see the most benefit since dx12 offers a more direct path of communication to the graphics card, less load on the cpu while it's also processing the game code. It won't reverse the order of performance hierarchy though. As sizzling mentioned, mainstream dx12 could be years down the road as games have to be programmed for it. In the meantime most everything else is still dx11. Dx12 will be a performance improvement for all users.

If you want/need the features of skylake it might be worth considering. Multiple m.2, sata express, ddr4 ram, additional usb 3.0/3.1 ports. If not then haswell is still just as viable. It also has m.2 and sata express depending on what motherboard you choose. Even though it's 'on its way out' its no more fate sealed than skylake is. Those who go with skylake aside from ddr4 ram and a few other platform changes won't have a platform/motherboard that's going to last them the next 3-4 builds just because it's the new kid on the block.

Following intel's past there will likely be one more cpu generation for 1151 (skylake) and then the next will be yet another socket/motherboard change. There really hasn't been any 'future proofing' with intel hardware for some time in that regard, the cpu and motherboard should more or less be considered a matched pair. There's generally no reason to upgrade just one generation of cpu (most people are comfortably skipping several generations) and beyond a single generation gap the next falls onto a new socket. For that reason skylake is just as 'stuck' as haswell.

If your motherboard supports the 8350 that would be your least expensive route and you'd still get decent performance. If it's going to mean upgrading the motherboard to something more capable for the 8350 it may be worth looking to build an intel pc and whether haswell or skylake you'll end up with a cpu/motherboard that should easily last the next 3-4 years or more. Haswell would be slightly cheaper and you could reuse your ddr3 ram. Skylake is the premium option since it's the new kid on the block and using all the latest hardware like ddr4 though there's not a ton of difference in games.

If you're in the u.s. now would be a good time to make these considerations with black fri and cyber mon around the corner. It could help you save a bit depending what deals are available, they'll probably have discounts on the 8350 and on both amd and intel motherboards as well as cpu/mobo combo deals. Just throwing options out there.