ok here are my suggestions (some of these are more so slight reworks as to how the item works with some items)

Immaculate Heart: tears that circle around you bounce
Saturnus: tears that circle around you bounce
Ocular Rift: chance to create small rifts with each bounce rather than just on the first bounce
C Section: shots bounce from being fired
My Reflection: shots bounce more on the way back
Dr.Fetus: small chance to explode with each bounce (excluding the first bounce)
Anti-Gravity: tears bounce while being held in place
Fire Mind: chance to create a fire with each bounce
Holy Light: chance to create light beams with each bounce
Head of the Keeper: Very small chance to create a coin with each bounce
Compound Fracture: bones split up with each bounce
Parasitoid: chance to create flies and spiders which each bounce
Jacob's Ladder: Chain electricity with each bounce
Pop!: tears constantly bounce

you can re-add synergies which got removed when flat stone was added to the booster pack

Proptosis: Tears only shrink when they bounce, effectively increasing range.

Fire Mind: Each bounce has a chance of creating a Red Candle flame.

Trisagion: Bounces. Creates a light beam with every bounce.

Dead Eye: Hitting the floor counts as a hit, rather than a miss. (So this combination becomes good instead of ruining the Dead Eye effect.)

Flat Stone (Duplicate): Tears are bigger, and produce a larger splash damage radius, represented by a visible splash.

Would be nice if you made all items in your synergies mods infinitely stackable with themsleves,for example Flat Stone can increase the splash % dmg and splash range by every copy

Dr. Fetus: The bomb bounces, sending splashes of Soymilk-sized tears randomly.

Decap Attack: Isaac's head bounces once before landing. The bounce and landing create Soy Milk-sized blood tears.