Re: Succesor to the Navajo, Chieftain, Metro
NMC:
The obvious replacement at the moment is the C208.
About 5 years ago at Oshkosh, a Piper VP told me he believed the era of the PA31/C402 class piston twin is over and out.
Any future Piper in this class (if ever) would be a single PT6 version.
As for the diesel engines; as DD wrote, some problems so far but the biggest issue right now is weight. The 350hp tielert is one heavy puppy.
As for a Metro replacement; the Embraer 120 at 30 seats comes to mind.