The setup I would build if I were doing a conversion ....
Base would be a Low-Loader Box Van conversion. Preferred vehicle probably a Renault Master fitted with twin Single Seats and 100% with Armrests

Seats would swivel to be able to rearward face the lounge area.
Layout would virtually replicate what I already have (which is kinda the reason why I have not really bothered looking into doing a conversion) - Which is front lounge, central Kitchen and rear 'en-suite' bathroom. And the reason I am saying 'en-suite' is that it is not small. It has toilet, basin, separate good sized shower cubicle, wardrobe and drawers and two bathroom cabinets plus room to dry and dress after a shower in a room that is the full width of the body. Very different to what most people look for in a Motorhome, but everyones different

Bed would be interesting ... try and do something like on an A-Class with a bed above the cab beds.
But what I would add - and a low-loader body would easily accomodate - is creating a space aka garage behind the bathroom the full width and height of the body accessible from the rear (probably using the original rear doors and making an internal wall around probably 2' - maybe 3' - inwards where the living accommodation will start.
Windows and Vents? Goes without saying really

Door? For sure

I hate the idea of having to use the cab doors to come in and out. Makes no sense to me but again, different folks, different strokes.
But something that I would like on a Door and seems to have gone totally out of fashion ... a stable door style i.e. being able to open the top half while keeping the lower closed. I am surprised these are not options, especially for motorhomers with dogs.
Probably be a gasless van - diesel heater, electric cooking and cooling and a nice big battery bank of maybe 1,000Ah (12V equivalent. Would actually likely go for a 24v 500Ah setup) with solar on the roof everywhere possible with sliding panels to deploy and layered when driving.
And ... just for a bit of fun and to try something different ... would
not go for a full monty Victron electrics setup but maybe try out the TBB Power System that Photonic Universe are selling. Saw it in the flesh at the Photonic stand at the NEC last October and looks smart and make a change from the usual.