Hey! So I've been working to make a few revisions to the existing specification (http://retrospec.cc), while adding a few connectors to the fray. I aim to make as few revisions to the spec as possible. People should be able to expect a rigid specification as they build. These revisions are directly related to feedback from fellow engineers.
I realized one thing in particular: there needed to be a connector geared toward those first trying out the spec. I decided on the DA-15 connector, since it is familiar yet slightly rare. The more 'pro' pins, in the new proposal, are found on DE-9 & DB-25.
Others communicated their desire for a longer-distance communication protocol present on these connectors. While generic RS-232, RS-422 & RS-485 were in the running, I decided on CAN bus. The bus will ideally provide the communication backbone for each group of modules. Consider CAN for both flexibility & long cable runs, and SPI/I2C for the simplicity & shorter cable runs.
There was another suggestion related to higher-current applications (i.e. motor controllers). There are connectors in the DB-25 form factor that feature a few large pins. These connectors are usually available via an online order and a slow boat. The connectors are a variant of older connectors which featured one or more coaxial connectors amongst the D-Sub pins.