Help defining trait function such that it implements IntoObserverSystem

[raybbian]

pub trait EntityBlueprint
where
    Self: Component + Default + Sized,
    EntityBundle<Self>: LdtkEntity + Bundle,
{
    const NAME: &'static str;
    type PARAM: SystemParam;

    fn register() -> impl Plugin {
        |app: &mut App| {
            trace!("Registering entity type: {0}", Self::NAME);
            app.register_ldtk_entity::<EntityBundle<Self>>(Self::NAME);
            app.add_observer(Self::build);
            app.add_observer(Self::hydrate);
        }
    }

    fn build(event: On<Add, Self>, mut commands: Commands) {
        commands.entity(event.entity).insert(Self::components());
    }

    fn hydrate(event: On<Add, Self>, param: Self::PARAM) {}

    fn components() -> impl Bundle {}
}

I have the above implementation of a trait where users should be allowed to define their own trigger parameters. However, Self::hydrate cannot become an ObserverSystem, with error message

1. `fn(On<'a, 'b, Add, Self>, ...) {<... as EntityBlueprint>::hydrate}` cannot become an `ObserverSystem`
   the trait `bevy::prelude::IntoSystem<bevy::prelude::On<'static, 'static, _, _>, (), _>` is not implemented for fn item `fn(On<'a, 'b, Add, Self>, ...) {<... as EntityBlueprint>::hydrate}`
   for function `ObserverSystem`s, ensure the first argument is `On<T>` and any subsequent ones are `SystemParam`
   the following other types implement trait `bevy::prelude::IntoSystem<In, Out, Marker>`:
     `IntoAdapterSystem<Func, S>` implements `bevy::prelude::IntoSystem<<Func as Adapt<<S as bevy::prelude::IntoSystem<I, O, M>>::System>>::In, <Func as Adapt<<S as bevy::prelude::IntoSystem<I, O, M>>::System>>::Out, (IsAdapterSystemMarker, I, O, M)>`
     `IntoPipeSystem<A, B>` implements `bevy::prelude::IntoSystem<IA, OB, (IsPipeSystemMarker, OA, IB, MA, MB)>`
   required for `fn(On<'a, 'b, Add, Self>, ...) {<... as EntityBlueprint>::hydrate}` to implement `IntoObserverSystem<_, _, _>`

Why is this the case? How should I define my trait implementation to workaround this?