It is indeed interesting, the whole debate about what can and can't be carried in certain ways IRL, and what would be unwise balance wise. But I do think you can consider the heavy crossbow to be a crossbow that is purely stronger than a "standard" hand-drawn crossbow, and that there is a niche for a physically stronger and more accurate "siege" crossbow with movement penalties as previously described. I wouldn't change the slots to 3 slots because 2 slots would allow you to carry a sidearm one handed sword, or extra bolts and a 0 slot sidearm. it may sound like fantasy lazy non-contributing pleb to say that the arbalest would need a new model and animation to reflect these changes and not actually have a placeholder made, but it's food for thought for anyone who likes the idea and actually has modelling ability.