This is an essential feature - being able to define a configuration of elements as a block/aggregate that can have multiple instances in the model/file. The functionality is similar to that of hotlink modules but blocks should primarily be seen as content internal to the model. Whereas hotlink modules can work on a larger scale e.g. differentiation between site and buildings, between different buildings and between different building volumes. Blocks would facilitate a fast and reliable workflow for repetition on a smaller scale within the model e.g. repetition of stories within a building volume, repetition of units within a story and repetition of bathroom/kitchen configuration over units.
It should be able to edit the block in-model and thus redefine the block and update all instances.
Changes to the definition of element types (not implemented feature) used in the block should redefine the block. (for element types see: https://archicad-talk.graphisoft.com/viewtopic.php?p=330207#p330207
It should allow for nesting of blocks within blocks.
It should come with a manager. Be able to count and select all instances of a block. Be able to see the relationship between nested blocks. Be able to insert blocks to model as an instance or as individual elements or as grouped individual elements.
Edit: Isn't this topic better placed in either User Interface Wishes or Construction Wishes?