Okay.. Glad to know that you're toying with some of the ideas. I'll keep adding ideas.
*LT = For a limited amount of time
*

= Not sure if easily possible
- Previously listed
• Lose the ability to run (Quite bad) *LT
• Slowed down (Quite bad) *LT
• Can't whip (Very bad) *LT
• Heal (Awesome)
• Poison (Reduce health [Very bad])
• Limit your vision (Turn room dark [OMG!])
• Restrict the ability to jump (Very bad) *LT
• Stun (Bad) *LT
• Make you super-hyped (Run faster [Good/Bad, depending on your skill]) *LT
• Jump higher (Good/Bad, depending on situation) *LT
• Hallucinations (Enemies that aren't there, blocks that aren't there [Totally lame]) *LT
• Weaker (Double damage taken [Very bad]) *LT
• Accidentally misplace an item (The shown up top-left of screen kind of item [Bad, or not so bad, depending on what item is lost])
• Accidentally drop/light a bomb (Bad)
• Accidentally use a rope (Bad)
• Find a bit of gold in said mushroom (Good)
• Accidentally Misplace some money (Bad)
• Sprite swap for the whole level (Quite bad)
• Gain a sixth sense (Spawn a ghost [OMG!])
• Find a random item in said mushroom (Good)
• Kill all enemies/damsels/shopkeepers that are not in "The Frozen Zone" (Ghost excluded [Good/bad])
- New +++ Adding rarity level suggestions * Common, Uncommon, Rare *
• Steroid effect - Deal triple damage (Rare + Very good) *LT
• Switch controls - Left = Right, Vice versa, Up = Down, Vice versa (Uncommon + Very bad) *LT *

• Sore back muscles - Can't crawl (Common + Bad) *LT
• Invincible - Invincible for about 8-12 seconds *Excludes Man Eating Plants, getting squished, spikes, and all other things that are insta-kill* (Rare + Great) *LT
• MGG's Idea - All treasure collected has double value (Uncommon + Good) *LT or whole level
• Sugar Rush - All enemies are slowed down while you maintain a normal speed *Excludes final boss* (Rare + Great) *LT *

• Increased Charms - All prices for items in shops are halved for the whole level (Rare + Very good) *

• Detect exit for that level - Compass arrow points to exit even if no compass is owned *Effect can be found even with compass* (Uncommon + Good) *

• *Already mentioned* Heal - Heal 1 HP (Uncommon + Very good)
• Refreshed - Heal 2 HP (Rare + Great!)
• Infection - Lose 1 HP every 30 seconds or minute for the whole level (Rare + Quite bad)
• Sore legs - Can't jump while running (Uncommon + Quite bad) *

That's all I can think of right now. But there's definitely more ideas ready to be plucked. Hopefully some that I mentioned here can be used.
