I would like to see a smallish rib (outboard) with a full width front cabin to sleep two, two single jockey seats and a rear bench seat that folds out into a bed. With the use of a tent/tarp that would join from the cabin to the A frame to make it an all enclosed four birth.
If you could make the single jockeys so the back rests can be turned around and have a table in the middle of the seating for meals or evening chats either under the stars or undercover with the tarp on.
A second battery that would run a cool box and lighting that wouldn't run the main battery down.
One of those gas powered stainless steel BBQ's that hang over the side, maybe near the helm position.
Toilet? I don't really like them but a sea toilet option would be a good idea, at least then you could use the rib anchored in a bay for the night.
A diesel heater (as poly said) would be good too.
There you go, nailed it