Many of us know that Minecraft allows for stat buffs/debuffs on items that will apply in certain places where it is equipped. If not here are the slot types and modifiers:
Slots: Any, Main hand, Off hand, Head, Chest, Leg, and Feet
Modifiers: Max health, Armor, follow range, knockback resistance, movement speed, attack damage armor toughness, attack speed, and luck
This feature alone would be great, but there is a way that could make future mods even more interesting. If you could, it would be great if we had modify by percent, set, modify by addition on [stat] for _ ticks or seconds and make this block stack upon others that are active (like two effects or item that amplify the same thing (I think you can see where I am going)). This would allow for new effects with independent effects. For example, you could have an effect that sets the player's health to 4, increases attack speed by 15, and slows them down by 150% while still having slowness having a different effect if it is added on. Yes, we do have most effects that can modify stats, but they are not independent, and this feature could change this.
NOTE: I do not code for Minecraft, so I can't help with that process.