If my Druid uses Wild Shape to become a large, Dire-class animal, he may weigh 2000 pounds or more. Is stepping on prone opponents a possible attack? What would the attack and damage rolls be?
It's not a Trample special attack; that would be applicable if the opponent was standing.
I just want to knock down opponents (for example, with a high-success Overrun) and then step on them.
Yes, that's the kind of thing you're doing when you attack - it's just that your size is already factored into your higher CMB, great strength due to size, and increased damage with natural weapons due to size.
As far as "something special" beyond that, no, because it's not needed - it's already "factored into" your characters attacks.
Get a really high Overrun bonus and take Vicious Stomp.
Adding to Redchigh's comment on home rules, if you wanted to stomp on then yes I would treat it like a normal attack. However, if you wanted to jump or fall on the target, I would treat it like a falling ceiling trap or similar. The attacker would end up prone regardless of the opponent’s reflex save. I would give the guy you are about to crush a reflex save with a base of 20 assuming the opponent is of medium size and in the prone and the attack is of large size. The modifiers would be:
-5 for standing opponents
-2 if kneeling or crouched
0 if prone
-5 for same size of the attacker, save no damage
+5/-5 for one size smaller than the attacker/defender, save no damage
+10/-10 for two size smaller than the attacker/defender, save half damage
+20/-20 for three size smaller than the attacker/defender, save half damage
Damage:
1D6 for medium
5D6 for Large
10D6 for Huge
20D6 for Gargantuan
40D6 for Colossal
Thoughts
Ok, look, you are obviously free to do anything you like with your game. However, knocking someone prone with a good Overrun check and then using Vicious Stomp to make an AoO against them for falling down is literally what is being described in the original post. No special rules are needed for this--it is already in the rules.