The Abuse Case for Hextech Rocket Belt
Meddler recently posted that HRB will apply a self silence for a short duration do reduce the combo potential of the item with assassins. This cuts down on most abuse cases but Ekko still has an incredibly powerful synergy with HRB which will probably break AP bruiser Ekko pretty hard.
Since Ekko's W is a delayed spell and his E is partially delayed Ekko will be able to combo the item absurdly well. He can cast his W then his E into HRB without really effecting his kit too much. He gains access to what is effectively a 1050 range gap closer with no serious downsides, in fact I'd say the stats on the item are fairly optimal for him.