A common feature in screenshot applications is to add a delay. A very common usage I have of it is for instance when I need to shoot an intermediary state of a window. For instance, when I want to shoot a menu contents: I would set a 2 sec delay, then quickly open the menu and wait for the screenshot. For fullscreen screenshots, it can be added programmatically before the dbus call, and for "screenshotArea" as well when a "selectArea" will be implemented (cf. bug 387721) in between the 2 dbus calls. But "interactive" manages everything from window selection to snapshot. So this would need to be added as a parameter to the dbus method (uint parameter for delay in second maybe?). Moreover doing so, it may be interesting to add a OSD effect to display a countdown before the screenshot (this countdown effect could of course be optional with another argument, boolean, to the method "interactive").
You could add a delay before issuing a dbus call. That's what spectacle does FTR.