It's been a while since last update, but the game is still coming along nicely considering I'm working on this part-time.
Here's a screen shot of the upgrade menu of the game (still WIP). You have your power-ups, items and character specific upgrades on the left, and then you will have a description of a selected item on the right. The exclamation mark on an item indicates that there is an upgrade available that you can afford. As you can see, I haven't decided on all power-ups and items yet.
Here's what I have planned now:
Power Coin - Increase multiplier
Mushroom - Invincibility for a short period of time
Feather - Warp ahead X areas
Heart - Start the game with an additional heart
The character with the white outline - To be honest I don't remember what this was.
Probably a resurrect?
All these are still likely to change.
Levels now support floor spikes that hurt the player:
Levels also support cannons that shoot fireballs:
My concern is that the game might be too easy. The only factor affecting difficulty is the number of enemies on screen. One way to vary this is with different type of enemies. Like the fireball there, killing it will cost 1 power coin, but the fireball won't drop any items once killed. Another enemy that I have is a ghost that, once hit, will respawn randomly on the level, and permanently die after 3 hits.
Also the upgrades need some thought. As all combat is resolved in one hit there is no need for more powerful weapons, so that limits the upgrade paths.
Oh, and another game that has been my inspiration recently: Power Stone.