One more point - history has shown that tanks (and even mech infantry when still mounted) cannot take urban areas. You need dismounted (leg) infantry to do that.
Given the nature of tank combat's evolution since WW1, I'd say there is a good argument to be made that Civ V tanks could evolve into 2-hex ranged units which cannot by themselves take cities. They would have offensive and defensive value, but only as far as penetration of lines and as standoff units. Put a tank in a city and it is a sitting duck. (Ask the Russians about their tank and mech infantry losses in Chechniya for a recent example.)
Thal, I'd like it if you would consider this idea for VE...that second generation and later tanks actually have range, and not be melee units. They cannot and should not take cities - not without leg infantry to occupy the objective.
This way you can replicate the tank battles of the Russian fronts and Western desert, with tanks taking on targets at range, even overrunning exposed infantry they have suppressed by long range fire.
Anti-tank units, on the other hand, would have very limited defense against infantry melee attacks (and be quite vulnerable to range 2 mortars), but would possess a staunch stand-off defense against tanks. So while the tanks and anti-tank units duel at range, the infantry has to approach the city on foot while being exposed to city defensive artillery.
The idea would be to make the player use these units in concert with one another. A city defended by an infantry unit, with an anti-tank unit on its adjacent outskirts to defeat tank long-ranged fires, would be one tough nut to crack - as it should be.