fzzf: It really depends. diskimage-builder provides a lot of elements [1], you can take a look and see what could be useful for your image. As per the documentation, the most minimal image [2] would have only the ubuntu or ubuntu-minimal element, and also the vm element, or else you won't be able to boot into your image (it does partitioning and stuff). But if you, like me, is interesed in reproducing OpenDev's setup, you'll want to take a look at the elements in the openstack/project-config repo [3]. They're more specific to OpenDev, but you can use them if you want to produce an identical image as the one used in OpenDev's CI

[1] https://docs.openstack.org/diskimage-builder/latest/elements.html
[2] https://docs.openstack.org/diskimage-builder/latest/user_guide/building_an_image.html#vm-image
[3] https://opendev.org/openstack/project-config/src/branch/master/nodepool/elements