AFAIK, without getting your feet wet in programming a custom .NET Web
part there's no way to customize this list.

There's a slight chance that some kind of messing with JScript and
FrontPage could get you somewhere but I haven't tried this myself as
of yet (although you're not the first one asking so it might jump up
into the priority of my list  of things to mess with <g>).

Although possible, there isn't any simple way to filter values in a
dropdown.

However, you can add a dropdown to your list  based upon the contents of
another list. What follows are the steps...

Create a new custom list, let's call it xrefAssignedTo. I would not display
it on the Quick Launch bar.

Add a new list item to xrefAssignedTo for each person  that you want to
eventually see in the dropdown on your issues list. Just use the default
Title column.

Modify columns and settings of your existing Issues list and Add a new
column called AssignedTo.

Choose Lookup as the type of information in the column.
Get information from: xrefAssignedTo
In this column: Title

Try to create a new issue  item. The new column  will be a drop down
containing the list items that you added. If you add other people  to the
xrefAssignedTo list, they will also show up in the list. If you remove
people from the xrefAssignedTo list, they will not only be removed from the
list but all previous references to them will be set to null.

Your other option  would be to add a Choice column directly to your Issues
list.