My next Intel build will be Z590 as I want full Gen 4 in 2021 and will want Rocket Lake since will be Intel's actual new Architecture. 
 
If needing something now then I personally would wait short while for Z490, 9600k isn't a very high end cpu to pair with a dark board, I would spend less on board and get a better cpu... again my opinion. 

VegetaCreeper
I’m thinking about getting an i5 9600k and pairing it with Z390 Dark.

Is this a waste to do now? Should I wait for new intel processors to hit the market?

If your rig in your signature is what you're currently using, then definitely wait. They just released specs of the next gen CPUs. The street launch I would be willing to bet is less than a month away at this point. You won't be getting that big of an upgrade going to a 9600K from your 8600K. You will however see an appreciable gain in going to a 10600K with double the thread count as that part will be a 6 core 12 thread CPU for roughly the same price. 

You should have any problems running new games on that Z370 system. If it was me i would hold off until Intel finally stops making 14nm+++++++++++++++++  CPU's.
 
Z370 should last you 2-3 more years at least.  I'm still running Z270.
 
 
 
 

VegetaCreeper
I’m thinking about getting an i5 9600k and pairing it with Z390 Dark.

Is this a waste to do now? Should I wait for new intel processors to hit the market?

That's a flat out silly combo.  You're better off with 9700K or 9900K with a less expensive board.
 
Heck, even just dropping a closeout 9700K ($249-$299?) or 9900K ($399-$449?) and some fast ram (4000Mhz or higher) into your current MSI Z370 board would be a better move.  That's a pretty good board with very reasonable high speed memory support.
 
Here's some Patriot 4133CL19 2x8gb for $99, sale ends today.  It's b-die.  https://www.newegg.com/patriot-16gb-288-pin-ddr4-sdram/p/N82E16820225143
 
You also should get an NVMe boot/game drive.

You only need to upgrade to a 12 core or something.

To the OP, EVGA Z390 motherboards are scarce right now. Amazon is the only place I could find it at MRSP. The one on Newegg selling it is basically smoking crack.
I would wait to see what the EVGA Z490 has to offer.
 
My thoughts (opinion only, comment if I got anything wrong please)
This launch along with all board manufacturers is reminiscent, to me, of the X58 launch (EVGA Classified X58 is still my current MB). Tried something new (tri-lane memory and intel poked the pooch on it). Intel is already saying that the Z490 will be compatible with the next Intel 11th Gen chip. Which is supposedly PCIe 4.0 compatible.
 
The  PCIe 4.0 compatibility from AMD's X570 chip doesn't require a Gen 3 Ryzen because the  X570 chip takes command of the PCIe 4.0 requirements and the boards will have re-drivers to push the signal on paths (more than a few inches that require it).
 
What manufacturers are currently adding to the Z490s for future PCIe 4.0 support is an alternate chip (outside of the Z490 chip) to generate/re-drive the PCIe 4.0 signal (review the ASRock motherboards). This chip would have to be controlled by the 11th gen Intel CPU (with a bios update most likely).  PCie 4.0 nativity may be part of the yet to be named next gen intel chipset.
Whether or not the CPU socket on the Z490 is setup for future PCIe 4.0 compatibility would depend on buying a board that supports that future upgrade.
 
This is the X-58 Intel chip set memory screw up part. How well will this PCIe 4.0 compatibility work on a Z490 board with no native PCIe 4.0 on the chip set is the unknown.
You are introducing a capability that is annexed from the main chip (also a separate failure point). Which means the next gen intel chip set would supposedly be incorporating that function and eliminating that annexed chip.
 
Personally, EVGA is the only reason I have not upgraded to AMD yet. I am waiting to see what their Z490 has to offer. I can wait as I am on an X-58. The X-58 was the 1st Intel build and a bad intro to intel. The previous other 3 were AMD. 2 ASUS boards, both BIOS chips bricked following their instructions (local guy did bios flashing that saved the boards). Last motherboard before intel was a DFI LANParty NF4 SLI-DR (layout of the EVGA DARK). I ran SCSI Ultra 160 on the first two until SATA came out on the DFI.
 
I like the EVGA motherboards both design and layout. It has been the easiest board to work with, updated and OC. I am not gaming currently on the PC so i have time to wait. But I sure do miss it.