Boss battles like Scylla, The Hydra, but the biggest example being Stormzilla! Stormzilla really feels unique because instead of your companions and you fighting you are piloting a mech suit.
I would LOVE if more Stormzilla like battles happened in the game.
Also I think that there should be an award for beating Stormzilla a bunch of times similar to the Gortez badge. Perhaps a badge known as "Mech Pilot" and wearing it makes you look like Stupendor X. Plus unlock Stupendor X as a companion? Just an idea