One animation I thought of is a "heart thumping" animation.
A few more
I thought of another animation. You could maybe have the heart where, out of the top of the heart comes little hearts.
3 animation suggestions:
1. Shrinks then enlarges to original size.
2. Heart breaks than unbreaks.
3. Transforms into a ball then transforms back into a heart.
these are pretty cool.
What about effects that emit from the heart? Like blood dripping or an indication of poison/decay?
Maybe you could add these as effects as layers over your current animations?