I think it would be great if Roundup allowed you to 
set dependencies or relationships on issues. You could 
specify that a certain issue could not be completed 
until another issue was resolved first, making one 
issue a parent issue or dependent on the other issue 
being resolved.

I don't know if this is something that would require a 
lot of change, maybe just an addition to the class 
list? Although things like blocking of issue 
resolution when a child issue is not resolved may not 
work.

A lot of other issue trackers have this feature, and 
it bothers me that Roundup is lacking in this regard.