Javalins would be neat for variety and would allow for some interesting concept-builds. I don't think they should be the same as spears though. Spears are different from javalins, traditionally shorter and lighter. If you want to make javalins a thing, I would make them better than a dart, and worse than a spear. Make it stackable, give it a weight of 1, and 1d6 damage as a thrown piercing weapon, with a size of medium, and a crit of x2. I would give it a "simple weapon" proficiency like a spear. This way, it is similar in stats to throwing axes, but the damage type is different and doesn't require martial proficiency. This would be mostly for flavor, and wouldn't destabilize anything.