The advantage of crossbows IRL is they require very little training, and also they can be quickly aimed and shot from cover with minimal exposure.
I would just buff them by removing or greatly reducing the aim/shoot lag.
To represent ease of use, you could literally make crossbow use skill up much faster than other ranged skills. I don't know the numbers but my feel is currently archery/crossbow/sling all take about an equal number of fails to master. Make crossbow use twice as easy to skill up (move branching points higher as necessary).
I would not buff their damage. X-D is 100% right about the limitations on crossbow power. But it's even worse, because IRL late medieval crossbows have the benefit of using metal limbs to pack a lot of tension in a small draw area. If you're using wood or horn or chitin, the same material that can be used for bows, you're not going to exceed the power a longbow can generate. So crossbows should not be more powerful than longbows as a rule; honestly they should not match longbows unless you have one made out of special material.
I would not give them knockdown. Throw gets knockdown because the mass of the projectile is so high, and the trade-off is range.