missing display name

[michigg]

most user do not set the first and last name, so the display name is empty.
This results in login errors on services which require a display name.

I think there are different option to get rid of this problem:

1. force the user to set his first or last name
2. if no first or last name exists set username as display name
3. if no first or last name exists set mail as display name
4. if no first or last name exists set a default name (e.g. John Doe) as display name)

I would prefer option two. Or are other options out there?