I like the idea of an offer / counter offer dynamic... I think it can be done with basically the current trade logic, just ditch the trade interest bar and replace it with text responses.
Offer a crappy "red" trade, CPU GM replies with "This is a joke right? Call me back when you aren't going to waste my time. *click*". Maybe a dice roll can be placed here where that team won't entertain your offers until the following week... I personally find that silly, GMs exist to listen to offers... don't think they would ignore any communication from a fellow GM because you hurt their feelings or whatnot... but I can understand people wanting it from a gameplay mechanic perspective.
Offer a respectable "yellow" trade, CPU GM makes a counter offer or asks you to submit an improved offer based on some dice roll. This CPU counter offer might be more than they'd accept in the end... they might be trying to roll you.... if you played hardball and re-submitted the same offer maybe they fold... but I would think almost all the logic required for the AI is already in place in today's game.
Offer a good "green" trade, CPU accepts or tries to weasel just a bit more out of you... depending on a dice roll.
In all three cases, you'd either be rejected, be accepted or have to go back to the drawing board to one degree or another. Not terribly more complex in the AI department, but more interactive for the user... just having counter offers would be a huge improvement.