Wayland needs... I think some EGL stuff to create OpenGL or Vulkan contexts in Wayland apps and a kernel API for graphics modesetting (resolution and refresh rate + misc related stuff). Also input support in libinput. Modesetting is perhaps the biggest piece of work, but I dare say not a huge one compared to having drivers for modern GPUs in the first place.

What's going to suck is that every major compositor +wlroots for the non-major ones is also going to need patches to use the BSD modesetting API - something that the BSD guys don't have direct control over and which is maybe a little out of their traditionalist(?) comfort zone.

Or are the BSDs simply going for a copy of the Linux KMS (kernel modesetting) API? Wouldn't be the worst idea, Linux finally seems to have something robust there after earlier attempts that were considered problematic.