Choosing best bus and protocol for ~128 clients over the wire
If you wanted a simple, existing solution, you could go for Onewire. It's made for hundreds of devices and high bus loads so you don't need buffers. The main drawback is low speed because of that.

It has only one data wire, so timing may be another complication. Various client implementations for µCs are available. 

But keep in mind the the drawbacks of a single bus:


a single bus is likely to fail miserably and how do you locate the culprit?
updating software through a slow bus is going to be a nuisance with 128 nodes.