Something I really like in terms of layout is a full (as in body) length sofa/bench which gives you the choice of sitting with your feet up or laying down chilling watching TV or reading or whatever. Both my front seats swivel - but slightly oddly do not swivel the full turnaround as there are two side panels that stop the full turn.
Sometimes I swivel them so I can use the base for putting stuff on. I have occasionally used the passenger seat swivelled with the Lagun Table (that is fitted behind the passenger seat) as a desk if I need to use my Laptop for 'proper' work,but generally not swivelled to sit on unless need more seating.
Mid-position Kitchen with a decent amount of preparation area with a door behind going to a large bathroom.
And what allows a good sized sitting area, decent kitchen and a big bathroom is the overcab bed.
This area is designed to be folded away and half the base pushed in when not in use, but I tend to leave it out in position as a full-time made-up bed and just have a standard height cab area as the extra height I would get is just not needed.
This Autotrail Cheyenne was my 4th "Motor Caravan", the first 3 being self-builds, improving and changing from what I had before, but this is the first Motorhome style van and I don't know what I would want any different in the layout within the same vehicle length. Extra outside locker storage would be good but I have got round that as much as I can by having a Fiamma Ultrabox 360 'Papoose' on the rear
And finally, obviously a key critera and the most important thing to consider for any layout is to make sure there is a place for the MVP (Most Valued Pussy)