Are you studying for some particular career area? Electrical Engineering? Mechanical Engineering?
Try to put in thought to all the reasons why this idea failed before.
Each connection is a cause of unreliability. Think about how to make the connections between the bricks as reliable as a solder in the factory.
Consider how you address the electrical resistance of each connection. What are they made of? Gold is best but expensive.
The current best connectors have resistance still significantly higher than solder.
As this is DC-DC there is no impedance, just resistance. However, each DC-DC converter relies on impedance within itself, so there is something to think about in terms of EM leakage out of the DC-DC converter brick.
Think about how to make them reliable in the long term, wear and tear, and how dirt in particular oils and salts off human touch will affect performance over time.
Think about how much extra material having one case around each module vs one case around a total product as now, the manufacturing cost and the added weight.
These issues has been why this idea has always died in the past, when a highly modular approach was considered, when it went through a rough manufacturing cost.
Consider for example how Anker mades it LC90 torch, it is a kind of modular so you could screw-on different USB socket types instead to make it say USB-C or Lightning. Does that screw-on method carry across better than plug-in?
Example from across the phone market:
Look how your drawing looks very similar to this phone modular design.
One way to be creative is to have the fewest bricks and place major components in each, hence say you'd connect only 2 bricks, not need 3+ bricks to make a working system. In a sense this is what Anker is doing right now, because the purchase decision of a charger is correlated to the purchase decision of the device (e.g. "buy new iphone, buy new charger") and the average lifetime of a phone is about the same as the lifetime of a charger, the cables last less so why Anker is separating the cable to be modular from the charger. So there is a lesson - limiting the choices via the USB socket.
One way to makes these reliable is have a limited set of cases which snap around the bricks and bricks must be in a limited set of size+shapes (I think your idea is this already yes?). Then you'd snap together a limited set of fixed size mAh cells (with no casing) with a limited set of DC-DC converters, with a limited set of plug types. Then if someone wanted to say change a 10000mAh Lightning to USB-PD then they'd change the DC-DC converter and the plug but not the cell. I think this is what you are thinking. The extra thought is about doing "just enough modularity" to make a gain.
If you do a BSc on Electrical Engineering, or ideally two degrees with your 2nd in Mechanical Engineering, then they teach these principals - and these the sort of qualifications Anker staff have now.