Some devices and networks need to know at configuration time what the data payloads will be. e.g. TSN cannot be scheduled without knowing max payload size, PLCs need to know max processing time to copy data around, etc. The current spec allows servers to defer defining array sizes until runtime, and in some cases the client will not know what it is getting until it receives the data.

For example, currently if a Variable has ValueRank = -2 the only reliable way to determine if it is an array or a scalar is to read the entire value and see how big it is. There is no guarantee that it wont change during the session or at the start of a new session.
Some additional constraints could be that when the Variable has ValueRank = -2 and it is instantiated as an array, the ArrayDimensions attribute must be completely defined and the ValueRank set to the number of dimensions of the array. If it is instantiated as a scalar then ValueRank shall be set to -1.

A similar problem would exist with arrays of variants, where each element is a different data type and size.

This could be handled as a new profile/conformance unit, or to create subtypes with the additional behaviors specified.
The advantage of a profile or conformance unit is that it could be tested during certification. The advantage of a subtype is that it can be used with any profile.