Random Thought: What if Frozen Mallet got BotRK's Active?
The pickaxe in Mallet gets replaced with Cutlass.
The Cutlass in BotRK gets replaced with... I have no idea. Brutalizer, maybe? Stinger for a super IAS/OnHit item? But it would probably get its passive buffed in some capacity to compensate for the loss of active. This would have relatively little impact on Ranged champions using Mallet, but suddenly Melee champions have the handy ranged slow active to help them get to (and stick to) their target. BotRK could be pushed a bit more toward 'this item is for soft-countering HP-heavy targets' or some such.
tl;dr: one slowing AD item is enough.