I like your thinking. ^.^ Different means of siege warfare would definitely be a good way to expand the game's variety, and digging is definitely something that is historically accurate. It would take time, but in the interests of not dragging it out, they could speed it up so that it isn't a huge amount of time. Basically, make it slow enough to allow the defender ample time to respond - but not so much that it drastically slows down the entire game.
Something else interesting would be air units - or rather, units that can go over the wall without climbing. I could certainly see dragons (if introduced into the game at some point) having such a means of siege, though it'd be a heck of a stretch to justify it for any other race.
"He uttered the word of curse, crush him with our Orcish iron!"