I think 75% AP scaling on Ionic Spark's passive would be enough to make it competitive, alongside the base damage, which could be lowered a tad if need be. Since the item doesn't add any AP of its own, there is that trade off, so you wouldn't see it replacing Lich Bane per se. This would basically put it in the AP/AS/On-Hit niche, which is where it belongs IMO.
And yes, I understand the base idea of Spark is to add wave clear, but note that Tiamat does the same thing, and scales with AD and AS, as well as Crit for the bonus damage, while Spark currently doesn't scale at all, making it a comparable item built for the same task, but doesn't work as well.
Since it deals magic damage, were it to scale, the scaling should absolutely be AP/AS, thus distancing it further from Tiamat. The scaling potential would also make it a good enough offensive item to warrant choosing it over the flat damage provided by Wit's End, provided you don't need the MR.