There's already at least one map that has built-in "fortress" defenses -- Discworld! If you control both Ankh and Morpork, your armies in both get a +1 def bonus (IIRC).
There have been a number of discussions regarding new uses for cards (too many started by myself). Not to be harsh, but your idea seems like it would require too much coding for no real benefit, and its complexity would ruin the simplicity of the game. I was flamed for suggesting new units: Infantry, Cavalry and Artillery. It was a well-deserved refusal; these new units would be too hard to explain to such a wide audience.
Now, trading in cards
already gives you a defense bonus -- more armies! It is more abstract and simple than your suggestion, but the results are similar (and IMO, superior). Think about it. Would anyone really cash in cards for 3 dummy armies that they can't attack with and disappear after one round? Or final protective fire from a "fortress"? The only way to win is to fight to win; fighting a defensive retreat in this game will not contribute to victory seven times out of eight.
Actually
having real armies to deploy is way better than getting some dice bonuses because they're more flexible (and lack is not about to "mess with the dice" because it would require the work-equivalent of almost a complete overhaul of the entire game's code).
Also, don't think of armies as armies.... When you have 3 surviving armies already in Thailand who've fought their way there and survived two invasion attempts against them, and you deploy 5 more armies there, consider it minimal influx of troops -- you're really representing the grizzled tenacity, veteran status, experience and eliteness of hardcore crack troops.
Consider a stack of 8 armies versus a stack of 3 armies versus a single 1 army. The 1 army would count as local police and fire with some armed volunteers. The 3 armies would count as a brigade of National Guard troops, or maybe some conscripts overwatched by a small detachment from a larger force nearby. You could consider the 8 armies to be an entire airborne unit, on full alert, say, an entire Parachute Infantry Regiment of the 82nd Airborne Division with a company from 10th Mountain tagging along. Although the manpower of these different stacks of armies is largely the same, the level of training, support and, uh, "eliteness" of these units is what is reflected by the size of the army stack.
Or you could just consider it hordes and hordes of cannon fodder.
