The monotone uniformity is the whole DNA of his laugh, there's very little I would personally want changed in that respect. Between the drawn out "ha..ha...haaaahhh" (as seen in the nightclub) and the "ho-ho-he-haha" Romero-esque delivery (in the helicopter), there's enough range between the two to tweak appropriately for each scene.
I actually give kudos for Leto coming up with a laugh wholly unique to his. Before this performance, I didn't think it was possible to come up with a new Joker laugh which hasn't already been heard before.