You know, that reminds me of an idea I once had for my MOD. There'd be this monster (I called him an "Asik") who behaves like a stalker, only he swims on the ceiling. And then there'd me this weapon the player has to get, a really long javelin-like thing called an "Asik Killer." (thus it's a melee weapon that doesn't require ammo) If you whack the asik with the asik killer just once, it will collapse to the floor and die. The asik is invulnerable to every other weapon.
I also had another idea for an ultra powerful weapon called the "God Angerer," which created a massive earthquake that damaged everything within a 1024 unit radius (including the player if you use it too long). It treats ammo like the Heretic's phoenix rod on tome of power, only you can prolong the earthquake as long as you want. (Unlike the phoenix rod, where there's a maximum 10 second duration).
And another idea I once had for a monster: a knight in full-plate armour, HP probably around 1000. It's very slow walking, but has a viciously powerful double-bladed weapon: one blade is a jagged sword, and the other is a long razor-sharp hook curving over its tip. Anyway, when damaged, it behaves like the centaur as it uses its shield to defend itself, only this knight can still move and attack while using the shield. What do you think about all these ideas?