I have some ideas:
Ice Weapons
What`s special? Ice weapons freeze the enemy, so they attack and move slower but you can`t have fire and ice weapons together.
Earth Weapons
What`s special? Earth weapons fossilize the enemy for a moment, so the enemy can`t attack, but you can`t have earth, fire and ice weapons together.
Fire Resistance Upgrade
What`s special? You live much longer while on fire, that`s usefull for killing the last enemy or something like that.
Ice Resistance Upgrade
What`s special? You can move and attack faster while you`re freezed.
Earth Resistance Upgrade
What`s special? The amount of time you`re fossilized is shorter.
Earth and Ice Challange
What`s special? It`s the same as the Inferno Challange but with Earth and Ice Robots.
What do you think about that?