To me, the most useful smoke unit would be one that was activated by the main gun and shot a puff of smoke from the barrel as the gun fires. I know Torpedochief was working on something like this but haven't heard of any progress being made.
Also a very high output smoke "generator " would be very cool for the more modern tanks. although we dont have very many of these.
smoke candle grenades would also be very cool but these would i assume fall under the " fireworks" catagory which as stupid as it sounds are illegal in NJ. land of the police state. Wouldn't it be cool if you could pop out one or two of these "grenades from the launchers and beat feet out of there. especially if the smke could block an incoming IR shot. just long enough to get to a more defend-able position.