Redmine

It would be very helpful if you could set dependency relations between issues, like this:

• Issue A depends on Issue B
  This means: A can only be started, after B is finished
• Issue B depends on Issue C
  B can only be started, after C is finished

  -> B and C are blocked
  -> A needs to be finished first

It's an m:n relation between issues, with possible conflicts:

• If A depends on B, and B depends on C, C can't depend on A
  -> That would cause and endless recursion, no issue could get started

With that dependency tree, you could plan easier, and hide issues, which can't be solved right now.