I just had an idea for a plugin that I think wouldn't be too hard for somebody with the knowledge to make it.
What it does, is it creates an adjustable spawn interval for the tank that determines a minimum and maximum amount of time before one will spawn an incoming tank at a random nearby location. (as in the tank is straight away coming for the survivors upon spawn).
The only thing that may be needed on top of it, is a way to disable tank spawns during crescendos or anytime when large infected hoards are incoming. Simply pausing the timer if it reaches 0 (spawn time) during this period will delay it just until the last rush dies out, then unpausing it and spawning a tank to top off the crescendo/rush would be fun. Also the timer start should of course be delayed until the survivors are so far from the safe room.
This would allow multiple tank encounters per map while also keeping the randomness of not knowing when or where they will appear. Adjusting the interval would pretty much determine how many tanks per map you would encounter in the long run.
Then to top it off, you could still use the force witch command while still getting tanks.
What do you think?