I am an engineer tasked with coming up with a new flight crew rest concept. I was after some ideas and sugestions from the people who use them.
Tom, with all due respect that's rather a "how long is a piece of string" question. It would depend greatly on how much space you had available and the anticipated flight durations. As a fellow engineer you know that it's all a compromise and it would be nice to say we would like a lounge, jacuzzi, space to entertain guests ... but it ain't gunna happen
However some aircraft are more space constrained than others and that will significantly affect what you're able to incorporate.
As echoed by other pilots here I think the basic "must haves" are;
quiet, dark, and ability to regulate temperature. Like any "good engineer" I'd suggest turning to established designs for ... err "inspiration"
Of the aircraft I've flown, I'd say the Airbus A330 crew rest is definitely one of the better I've used. Some features to note in addition to the above must haves are; comfortable seat, long lie flat bed that folds down easily, cupboard for bag storage, entertainment system, phone, and good reading lights.
Major changes I'd make post 9/11 would be to have it on the flight deck side of the door if possible, and if the aircraft size allowed, dedicated flight crew toilet. In other words when flying with a heavy crew they never have any need to go past the flight deck door at any stage of the flight. I haven't been keeping up with the requirements in this area, but last thing I heard this was being mooted as a requirement for security purposes in future designs. No idea if that has become anything beyond a rumour, and perhaps you could advise? It would definitely earn bonus points with regard security even if not a regulatory requirement.
Hope that helps and good luck with it. I've done my fair share of ultra-longhaul and sure would appreciate good design effort in this area!