I'd love a tiger pit trap for this game. It would resemble a patch of grass and when a pirate walked over if they would fall in and perish. Brutes and lower would succumb to their pointy death but jugs and heavy gunners would walk right over it. Also - to keep with the spirit of the game, thieves should be immune to all traps.
Another trap if like to see is a very large, albeit sacrificial, trap. A cache of powder kegs that also destroy any buildings within range. When deployed properly you could decimate your attacker in one swoop
I'm all for any traps featured in Swiss Family Robinson. And, of course, the sharks with friggin' laser beams attached to their heads.