Sven
here's another thought for the GUI structural time series modelling options. Currently the Basic Structural Model only has a seasonal cyclical element. Were this to be flexible and allow a user defined periodicity/frequency for annual series, then longer term cycles could be incorporated into the BSM. Admittedly, a trig cycle can be added into the exogenous variable group, but it will be deterministic, whereas a stochastic cycle would be useful (as most of those I have looked at are not completely regular).
Brian