Why not allow creation of degraded arrays? These have creation syntax such as:
mdadm --create /dev/md0 --level=1 --raid-disks=2 missing /dev/sdb4
So, instead of needing two devices, which webmin currently requires, only one is needed.
What is the purpose you ask? Well, this allows, for one example, you to convert an existing single disk system to mirroring by adding a second drive. So, the idea is you create your partitions and raid and whatever on drive 2, and, copy everything to it, and, then, after you boot from it, you simply set up drive 1 and add it to the array thus rebuilding drive 1, thus having raid without adding another drive for the conversion.
It's a simple enhancement.
However, I would also suggest supporting further mdadm flags such as marking a device as faulty, and even remove.