Thinking a little outside the box, your extra channels could drive "electric pots" ( MCP413X )
You control 2 virtual pots by sending them serial data. This would take 2 Arduino output pins each.
One channel produces a "clock" voltage and the other produces a "data" voltage.
On the receiving end, these two channels of analog voltage can be treated as digital, probably thresholding first since you will not have a full 0
or full 5v coming from these 2 extra analog channels. The "clock" and "data" now go into a shift register chip, which latches the data inbetween chars.
These bits of the shift register could control hardware directly, or be read by the Arduino and used to control something else.
It would be like you're spelling out a letter in code by quickly flipping a joystick up and down, using 0 and full only.
I'm not saying this would be cost effective or very fast.
It is, however, pretty unique, novel, and capable of a huge number of digital bits using only your 2 channels.
You might need a "sanity check" - if you are sending out say 64 bits of data and receive only 60, you don't latch the data.