If I understand Stan correctly so far (emphasis on the 'if' and 'correctly' here), I don't think there is much extra headroom with the current version I am fiddling with using the CD4070 ICs. There is more load capacity with the 74xx86 ICs Stan first mentioned, but as he also pointed out those are only 5 volt. Seems to be a slight drawback to every plan, without adding a bunch of extra components. Again, as I am understanding this anyway.
However, I think Stan had the additional output loads in mond with the transistor and MOSFET versions above. Those are not CA - CC interchangeable by jumper selection, they would need separate PCBs for CA or CC. I am not sure of how much extra load they can handle, I'll have to look that up on the data sheets. (I don't know (or completely understand) all the components like Stan and GRJ do.)
Then there is the PIC chip that GRJ pointed out, which I also plan to start learning about and fiddling with. This would provide features more easily than using the ICs and also be easier to modify, possibly without adding too many extra components. But again I need to study the data sheet for this one, as the PIC chips are completely new to me. I have fiddled with an Arduino Uno, but there's a lot left to learn there as well.
I would certainly be interested in the 3rd party traffic controller you used, what all it does, and how it functions. Sounds very interesting to me. Did you completely replace the MTH controller with the 3rd party one or did you use them both together? Since this is really a learning project for me as much as anything, I would be interested in learning as much as I possibly can.