Aurigny tried the Twin Otter in the early 80s, but found the maintenance costs to be too high due to the high number of cycles on the PT 6A engines.
If you do 20+ flights a day, the cyclic lives of the turbine discs is soon used up.
They reverted to the Trislander for the inter island services.
Any replacement will have to take this into consideration.
I can't think of a piston engined aircraft to do the job other than the Trislander.