Obviously nothing less than a RTX 2080.
Obviously...although, we must concede that an eGFX 650 and a Vega 64 is going to run about $900 out the door and with a decent active Thunderbolt 3 cable in order to make sure you have some length to work with on a desktop and still get that full 40Gbps bandwidth.
I am curious to see if the RX590 ever materializes or the Vega nano. I can build a Radeon RX580 (Sapphire Pulse) with an eGFX 350 for around $450 separately or $429 bundled together by Sonnet Technology.
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My response was in regards to thermal constraints ruling out dedicated GPU in the Mac Mini. It was not a price comparison. I simply showed what I put together in a similar sized box without thermal constraints, which happened to only cost $550. I put that little box together myself with a dedicated GPU and don't have thermal issues. So if Apple wanted to they can easily put a dedicated GPU in there.
A 128GB NVMe SSD is $29. I paid $20 for the 2.5". Samsung 970EVO is $90. Who needs Thunderbolt if there was a GPU in there? And yes I have experimented with eGPU on an Intel NUC7, which to my surprise worked with hackintosh.
And what you listed above is not a Mac Mini replacement as it's a good sized tower. I can build a box in my sleep, been doing it for yearsI just sold an mITX build in a 9" wide x 7" tall x 11" deep case with a full sized GTX1070 and i7-7700K in it. No thermal issue there either.
I never criticized Apple's pricing on the new Mini. It is not completely out of this world for not having to putz around with hackintoshing. My only criticism is the inability to add a 2.5" drive. Having dual physical drives in any desktop regardless of size is a must for me. I've also built even smaller PCs based on mini-STX sized motherboards, still with dual 2.5" drive bays.
Thunderbolt 3 serves many other purposes than for the eGPU. I have a SanDisk Extreme Portable SSD (USB-C Gen 2) with my 2016 MacBook Pro, which while not fast enough to test the boundaries of Gen 2's 10Gbps, I can always purchase a Samsung X5 Thunderbolt 3 SSD should I need that kind of speed for editing 4K video. I can add an eGPU, or more for rendering out video. I could purchase a PCIe expansion chassis and put a Universal Audio UAD-2 PCIe card, I can add one easily (I use an Apollo Twin Mark II right now). I can add 10Gbps Ethernet via a number of different adapters if I ever need that particular connection method. I can add a Dock for more USB 3.0 ports, an SD Card reader, eSATA, HDMI and/or DisplayPort. If I ever move to a Mirrorless camera that uses CFast or XQD cards I can add a reader for that and get the best performance there is for transfer/ingest. I can attach to a QNAP NAS and work in a group on 4K video or by myself. I can hook up a Thunderbolt 3 RAID for the same thing. I can literally scale up or down as necessary.
Again, Thunderbolt 3 makes the Mac mini and the MacBook Pros into Swiss Army Knives, but I get to choose the model knife I want and then change to a different model whenever I need to add different tools.
If those things are nothing you need then the Mac mini might not be for you, which is okay too.