Wow does this actually sound like a d2 ripoff?

K so here I was thinking of ideas for the 3rd act of the RPG for the DCM. Thinking, thinking and more thinking. Couldn't really come up with anything. The terrain is like a giant dungeon and this is where you fight the final boss (quiet you, I'm not giving away the RPG) and I came up with the idea that you need to kill all the monsters before you can fight the final boss, I started to elaborate on this some more, and made it so that a specific type of monsters need to be killed in order for the big bad guy to fight you. I started talking to my friends about this and they are like "Wow, can you say Diablo II rip off!?" I read over what I just said and yeah, it's like act 5 of diablo 2. Now I'm trying to decide if this is what the final act should be about, or if it is based on something else. The terrain is still going to be a giant dungeon though, that part fits the storyline.

Now I need ideas of what should happen before the final battle actually begins. I don't really want to use this idea because I want to stay away from D2 as much as I can.

Wouldnt worry about ripping off D2. Its not the most origonal setup in the world anyway. Just make the game fun.