So you want to give Peach a system similar to the one I've heard Ike is getting? Hmm...
All of Ike's moves get powered up in "No Sympathy" mode, but he damages himself with each one, and loses the effect if he uses Eruption.
How should Peach's Heart mode work... Here are a few random ideas:
Her Final Smash consists of putting everyone to sleep and spawning a bunch of Peaches. Sleep and Recovery. Considering that...
How about this: When Peach lands enough hits with Heart attacks, she enters... Heart mode? Ha, think of a better name later. Anyhow, while in this mode, she could have some of the following buffs:
- Her "Lalalalalala" taunt puts opponents to sleep. Very short range.
- A fire effect is added to her Frying Pan, and it does a little more damage. No extra knockback.
- Her Golf Club sends opponents farther, at more of an arc trajectory.
- Her Tennis Racket sends foes away at a lower angle.
- Toad's spores do a little more damage and have higher knockback.
- Heart moves heal Peach a little whenever they hit during Heart Mode. Big hearts only, like U-Tilt and Side-B.
- An electric effect is added to F-Air, along with slightly higher knockback OR damage.
- One of her throws is weakened, but gives the Flower effect.
- Foes bounce off of her parasol taunt and Up-B, taking slight damage.
- A fully-charged F-Smash swings her umbrella. Weaker than the other 3 items, but fires a large gust of wind.
- Her "Peachy" taunt spawns a peach.
More powerful buffs that would end Heart Mode if used:
- Hold Down-B to draw a guaranteed Bob-omb.
- Hold Neutral-B for Toad to fire a Poison Mushroom instead of spores, OR fire differently-colored spores that shrink the target.
- If she grabs an opponent, does not hit pummel them, and releases them, both the foe and Peach are healed fully OR by a set amount.
Disadvantages to Heart Mode:
- Perhaps it should be a choice to use this mode, instead of an automatic thing?
- Maybe Peach should be lighter, take slightly more damage from each hit, and/or have another weakness.
- Heart Mode should be lost when Peach is KO'ed, unless the buffs aren't that strong or the weaknesses justify them staying.
I know most of those ideas won't work -- I don't even play as Peach often -- but maybe some of them will?