Currently FM3 does not allow other default, question is if we really need that, as we have no usecase yet. @Lukas, how is it in Magritte?
In Magritte default values are nil by default, also for numbers and booleans. Attributes can have a different default value. As Toon explained I have examples where the default value for a boolean is true or the one for a number is 17 ;-)
Cheers, Lukas