Two thoughts.
Let's get it fully compatible with x10 rf first. Both regular signals and security signals (kill the ninja project).
Then make it work with some off the shelf sensors, be it manufactured under the x10 name, or some other. I like the idea of having digital collectors on it, but that may be a small niche. Water, lux, humidity, and temperature would make nice additions contact switches and PIR.
I don't expect a Companion module to be much more then RF if it comes to being.
However I would expect it to handle
all X10 RF even those rarely used codes.
I hope if something else is added it is something X10 has had experience with in the past which would cut down development time.
The owner should rethink that. Multiple companion modules with limited feature sets would be easier to support and would generate more profits.
Multiple companion modules makes sense
Create one with the tech they're familiar with first.
If they wish to add something to a RF companion module make it something simple.
I don't want to wait another 3-5 years for RF, thought I do have a way of handling that currently.