Acctually a shield bash wouldn't be simple at all. It is an action that isn't included in the NWN game mechanics, and the NWN feats and such are balanced without it included. I'm not sure a shield bash action (or feat that allows the action) would be really usefull or desirable in NWN even excluding the technical issues about creating such an action. You've got knockdown and you can attack with your weapon.
What would be nice would be charges, bull rush and ready acitons. However things like that, that deal with the core combat mechanics are almost always impossible to script in a desirable manner. They're possible, but making them work just right often isn't.
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There's support set up for armor spikes and a bunch of fancy stuff in CNR (paraphrasing forsetti) but it was never put in, and at this point would require a considerable balance and desirability investigation to mesh with the current game.
