Knockdown is a very powerful feat. I was thinking that tripping people over, pushing them down, etc, should probably suffer from a damage penalty (like half damage, or something).

Any comments?

This is unfortunately not viable within the NWN engine restrictions, and we currently lack free and able hands to make a suitable NWNX plugin.

Furthermore, I'm not entirely in agreement that Knockdown is overpowered - though certainly it is very useful!

No.