I like the idea of getting ad after a while, I think it seems ok. just not to keen on the idea of getting true damage on every bruiser. so here is my suggestion.
"After 4 seconds in combat, your first attack against an enemy champion grants you 3-20 (+10%AD) adaptive force for 4 seconds, this bonus is stacked for every 2.5 seconds you remain in combat.
Melee Only: Damaging enemy champions refreshes this buff's duration."
Any feedback? Numbers might be off.