Might as well bring back the Mk37.Hrm. Subs only.
I really think we're at the point where you need heavyweight speed and range for airdropped and surface launched ASW torpedoes. Modern nuclear and lithium battery subs can have enough acceleration to outrun/exhaust even a Mk48 if you're not close enough at launch.
So, I'm thinking a ~1500lb ASW torpedo with reduced warhead and Mk48/Spearfish engine and fuel tank.