Would you add a slit to this dress?

It may not work so well to have an actual slit since the flow of the skirt could change and maybe not keep the slit well positioned. One option that might be innovative would be to add a bit wider sheer lace inset in the opaque lining without upsetting the flow of the skirt.

You would need a seam in the front of the gown to add a slit. I wouldn't add one in this gown.

I personally wouldn't add a slit in this dress, I think it looks beautiful as is.

That type of dress structure and fabric needs a particular way of kicking out while walking so fabric doesn't collect between the legs. If you add a slit, this may cause other problems and prevent you from walking gracefully down the aisle.