The 4 second wind up time before you can activate the rune is extremely annoying in lane and makes the rune feel really clunky to use. During laning phase most Melee champions only do short trades so nobody fights long enough for the rune to activate. This means the only way to consistently activate the rune is by constantly hitting minions, which at high level play is extremely bad. You can't just randomly hit minions and ignore wave management, it literally results in you losing lane.
This effectively means the only time we can actualy use the rune during laning phase is if we are pushing towards the enemy (since we have the rune charged off hitting minions), in which case the enemy shouldn't even be trading with us (because they won't have the rune charged up).
We've not even discussed team fights yet, but you should know that a 4 second windup time is extremely problematic. There is no way that a bruiser can activate and then maintain Conqueror during team fights, heck I'd put odds on ADC's being able to maintain it better even though the ADC's can't fully utilise it due to the added CD.
* Conqueror shouldn't be able to be charged up on minions
* The Wind up time before the rune activates should be changed to 2 seconds for Melee champions but should remain 4 seconds for ranged champions.
* Alternatively the rune could activate after a certain amount of ability casts/damage instances within a small time window (similar to phase rush/electrocute).
This way it is a lot easier for Melees to activate the rune during laning phase & team fights. Other Keystone Runes with similar wind up mechanics (PTA/Electrocute/Phase Rush) don't even take 2 seconds to activate, let alone the 4 seconds it'll take to activate Conqueror.