Since this is on the MMA forum, I'm thinking the thrust here is sport combat. I am confused about the question of applying a submission hold after the takedown. Why else would you take him down? Just to score on the scorecards? What about ground and pound? That sounds punishing.
I'm not into MMA, BJJ, or wrestling. But when I see the words, "punishing takedown," I visualize the punishment directly resulting from the takedown. In other words, the takedown itself is punishing. For example, the "piledriver" where the opponent is slammed onto the matt head first (although I've only seen this on pro wrestling.) I did see Matt Hughes win a bout with a body slam. Ouch!
To leave the realm of sports and look at actual combat, a punishing takedown could be hooking the arm and taking the guy down, keeping hold of it so it turns into an arm bar as he's falling. If you then cinch up on his arm, his own weight going down will aid in snapping his elbow - the takedown supplies the punishment. There are takedowns that can rip up a shoulder, wrench a back, break a wrist or even neck as the opponent is on his way down, or as he hits the ground. Fight over.