I know a lot of work has been put into the finale explosion, but it seems really gimmicky. The rocket is over-used (I mean seriously, there are other doomsday devices besides nuclear and/or payload rockets). The fueling line has a massive hand-operated coupler on the end, which makes no sense at all. And the fan that pushes the bomb, while it is ok in its cartoony-ness, serves no apparent purpose.
If you want to consider doomsday devices or weapons of mass destruction that haven't been done, you could do a massive cannon, a bombing plane, a flying saucer, a time machine/portal, a particle smasher (the hadron collider), some kind of hulk-like bio-experiment, a submarine, or an airship. But, please, Valve has already covered lasers and rockets and earthquake machines.