Many Mini-ITX motherboards aren't exactly standouts due to their features or overclocking features, as the lack of space what can be achieved. The ASRock Fatal1ty Z270 Gaming-ITX/ac, the highest positioned ITX motherboard on the market and the first Z270 Mini-ITX motherboard we've tested, proofs that space does not have to be a limiting factor.
The Z270 chipset doesn't add much on Mini-ITX motherboards, compared to (micro-)ATX motherboards. The two main differences between Z270 and H270 are the support for splitting up lanes (in practice this means SLI and CrossFire support) and overclocking. Multiple PCI-Express slots won't fit on a board like this, which is why the first feature is rather pointless in this case. Support for overclocking isn't that useful in most cases either, because the power circuit and MOSFET heatsinks are simply not good enough.
This board is an exception. The ASRock Fatal1ty Z270 Gaming-ITX/ac has a similar feature set compared to most ATX boards, in addition to a beefy power circuit for overclockers. Of course you'll be limited to one graphics card, but this board offers everything you could expect from an ITX model.
Buy ASRock Fatal1ty Z270 Gaming-ITX/ac
At the time of writing the Fatal1ty is unfortunately not readily available: no shop or distributor has it in stock. The price is also quite high: about 265 dollars.