It would be a very good idea for Firaxis to release a mod that adressed the Spy system, its been recognized sinse day one as the games "weakest link" in contrast to previous Civ games. Its also a flavor contrast to the first expantion which was focused on war, a second one focused on Espionage and Stealth would have be different enough to be readily destinguished by casual buyers that could be played up for better sales.
But given their previous patterns and my current plans and pace I'll have twice their content before their expantion goes gold, just like we moded in our own Great Generals before Firaxis made them (and I would argue the moded ones were more interesting in their abilites as they projected an "aura" that improved all units around them). We have something near the equivilent of a whole free expantion pack or two worth of content modification here with an exponentialy incressing rate of moding.