Is wall collision detection working out well for you? I know sometimes there can be issues when making side-scrollers like being able to scale walls if the detection isn't just right.
Also, you should definitely try out a game called Spelunky if you guys are looking for more inspirational ideas. Spelunky is a roguelike set in a very deep set of caverns with varying themes and a lot of items that make a huge difference every time you get an upgrade. Might give you ideas for what kinds of weapons and items would be useful to include.
Here's some footage, along with hilarious commentary by my favorite user on youtube: