The finale is thematically awesome, but the gameplay isn’t. Just hordes of slave rats after you get in. Obviously it needs more, but I had a further thought that uses an in-game mechanic for a unique finale:
Whoever is rotating the cannon or carrying the cannon ball, works as if they have Taunt. After all, aren’t the Skaven going to desperately want to stop the cannon from firing? Stopping that should be their overriding goal. Players could keep taunt for a second or two after they stop doing the objective, or it switches if someone else starts doing it. Would be a unique finale, make doing the thing more than just holding E, and add a little more difficulty.