Right now there's a shortcut made by Konsole, but I think it'd be more intuitive for there to be a general shortcut to open the *default* terminal instead.
The Ctrl+Alt+T shortcut is set by Konsole itself, so this would require redoing some things. To make it work, we would need to un-set that, add a new action (owned by who? Maybe Plasma) to open the default terminal app, and then bind Ctrl+Alt+T to it. And then accept that there would be a shortcut conflict during the between when Plasma adds that shortcut and when Konsole removes it, because they're on different release cycles. And also that people who have just Konsole installed don't get this shortcut by default anymore--though maybe that's a good thing as it's kind of odd for an app like Konsole to register itself with a global shortcut even in other environments IMO. People probably want the same "Ctrl+Alt+T to open the default terminal app" there too.