I think the newsboy cap style compliments a leather jacket very nicely.
There's a good photo on Aero's Facebook page of a guy in a cap and leather jacket...
The Failsworth caps Aero sells aren't as wide as most of the vintage repro options out there, but they are very nice - more of a 50s than a 30s cut, IMO. And insanely cheap by comparison.... buy both brown and grey, they look great with leathers. I wear mine all the time when I'm going more casual, with denim, or when it's too windy for a brimmed hat.
I tend to think that, when the outer garment is of jacket length, the best accompanying headgear is a cap; to me the trilby/fedora looks better with longer coats like overcoats or trench-coats. Caps also go well with some but not all long coats.Sold on the newsboy for a super casual look.