I agree with the points about the potential of seaplanes at Dundee, the sea is right beside the airport, what more could seaplanes want?!
Also agree that long term the runway could be extended, as long as it was not just for the sake of having a longer runway.
Not so sure however it is necessary or viable a new terminal would be. Even if Dundee had the level of services it should the terminal would probably be sufficient. If services did increase then the provision for emergency services would adjust accordingly and the apron would probably be increased to cope, as it was recently.
Shed Rule