The design question I keep coming back to is “How do I create a reliable interlocking system for blocks?”  My criteria for a robust and useful system are:

1. The interlocking system should allow interconnections in three dimensions.
2. The pieces should snap/interlock together reasonably well/easily.
3. The pieces should stay assembled reasonably well.
4. The pieces should snap apart reasonably well/easily.
5. The pieces should not require additional tools to be assembled and disassembled.