(Standard disclaimer apology if someone has already proposed the same idea)
With more players available and trained in Martial Imagination and Piety, Rook attacks are usually put down quickly. There is plenty of time for a mob to teleport in, stun the Rook, load up the shrine, and call it a win. That's just fine, like a well-developed immune system wiping out a virus before it can get established. I like the idea previously mentioned of Rooking 'spreading' if untended, but it's rarely untended.
Instead, I'd like to suggest a more personal, more vicious approach by the Rook: targeted, immediate attacks. The Rook swoops in, and settles on a tree, ripping away its fruits or bubbles, wearing it down bit by bit, eventually killing it dead if not stopped in time. The nasty black thing drops onto a piggy, and pins it down, terrorizing the trapped creature until it eventually flees. (Not going for brutal violence, just the cold, cruel heartlessness that the Giants' opposition should show.)
This should occur when just one Glitch is on the street, with appropriate frightening music. They will know that unless they act, something is getting Rooked. If that Glitch has MI, they might be able to create a solo stun and drive the Rook off (lower total requirement than a full street attack). Perhaps with Piety, they could summon the power of the Giants and deliver a nasty ZOT! in person. If they're not capable of dislodging the Rook, they can call for help... but fast!
My idea is to make Rook attacks a bit more personal and horrible, rather than a odd sort of party, the way they often seem currently. The two types could also cooexist just fine, and possibly if a Rook goes unbeaten after a targeted attack, it could move on to a full assault, or just keep hopping from resource to resource until someone fights back.
Sound interesting?