Upcoming MATE image from Martin will boot on the Pi 3B+ and will use the Ubuntu-provided linux-raspi2 kernel. It will probably come in armhf format and not arm64, but you can enable multiarch by following the instructions on the Ubuntu Wiki if you want. X11 fturbo acceleration will also be present in this image.
I just hope the image won't come with all the Raspberry Pi bloat and PPAs that the Xenial image did. I also hope that each time a new bionic point release is made, the Raspberry Pi image is updated along with it.
Good luck Martin. Tomorrow when I have time, I will experiment with livecd-rootfs to see how I can create my own Ubuntu images for other flavours such as Kubuntu and Ubuntu Budgie.