The first 7 suggestions are from my combat mod idea on the Mod Development discord server.

1. Axes have slightly less reach, whereas hoes can hit from farther away,
2. Crits do more damage depending on how far you've fallen,
3. Shovels do slightly more damage to unarmored foes, whereas pickaxes have some sort of weak armor piercing,
4. Multishot and Piercing compatability on crossbows,
5. Stone tools do more knockback, wood tools do slightly less,
6. Fire damage and attack damage have different damage cooldowns, fixing that annoying no-damage-when-mobs-are-burning bug,
7. Shears could do more durability damage,
8. Reduce knockback dealt, and knockback taken while sneaking,
9. Crossbows could shoot firecharges, and snowballs, (or perhaps anything a dispenser can),
10. Piercing crossbows could pierce through wither armor, dealing half damage,
11. Tridents with Loyalty 3 could return if they've been thrown into the void,
12. Dragon breath bottles could be placeable, allowing for an easy instant damage cloud,
13. Hitting or shooting an Ender Pearl while it's mid-air would damage the owner and cancel the teleportation,
14. Pumpkins could protect from falling dripstone, whereas turtle helmets could protect from impact damage (kinetic energy),

I'm not sure if these would fit too well with the mod, but that's up to you. Keep up the good work :)