Giants are extremely cool mobs and serve to enchance the RPG potential of a server and I think that it would be very nice to see a plugin that allows specified amounts of giants to naturall spawn. Other possible capabilities could be for them to live in groups and make protected perimeters such as the giants of Skyrim. Many plugins have offered to tweak the characteristics of mobs to try to add "new" mobs but I think they have overlooked a very cool, built in mob. I know they were "removed" from the game but the essentials plugin can still spawn them so server sided code is still somwhere in there.
You can make this feature with the plugin VariableTriggers Look into it and check out the VT user forum http://dev.bukkit.org/server-mods/variabletriggers/forum/
