Conqueror's ranged usability needs to go up if its going to be available for ranged use at all.
The extremely large majority of ranged champions could not make good use of it even before the nerfs. Of those that could, the primary users were abuse cases, like Ryze and Cass.
Now instead of Conqueror being balanced for ranged, only Cass and champions with literally no good keystone for their playstyle use it. 
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If Conqueror is going to be allowed on ranged at all, it should be somewhat viable on them. It is not in its current state.
#Rather than just nerfing Conqueror for ranged, usability for ranged should have gone slightly up alongside it.
Conqueror should be changed to 2 stacks per auto attack, with a ~1-2 CD on ranged auto attacks adding stacks. Auto attacks while on CD would only refresh stacks.
Melee abilities should apply 2 conqueror stacks, while ranged provide 1.
#Melee stacking is unchanged.
Ranged stacking becomes more about auto attacking, or taking time to stop and auto, many of which are slow without attack speed. There is a CD on this to prevent ranged from abusing melee early too much and to prevent ADCs from being the best users of the keystone.
Ranged abilities providing one stack reduces the ability for ADCs and former abuse cases to stack it too quickly alongside auto attacks, and prevents mages from utilizing it better than melee champions.
Finally, it lowers potential stat-checking early by ranged champions by requiring auto attacks over a longer period of time, rather than relying on abilities, so minion aggro is a much larger factor in playing aggressively with it.
For Cass, as an example, this requires her to stop and auto attack rather than just spamming E 4 times after hitting Q, slowing her down a little in early all-ins. Keep in mind she would not be shrugging off minion damage anymore either because of how much longer it would take her to stack it.
#These changes would not necessarily be more balanced initially, but they would be MUCH better baseline for balancing the keystone around ranged, rather than making it unusable when the original intent was to make it usable for ranged.
######There are then 5 different tuning levers that can be reasonably adjusted for ranged to make it a decent, but not overpowered pick: stack duration, AA CD, AA stacks+required stacks, ability stacks+required stacks, and bonuses granted. If specific cases are an issue, their AA wind-up could even be modified.
