The C-5 Galaxy carries 73 passengers in a windowless cabin. If passengers want to know what is going on outside, then it wouldn't be difficult to run a feed from an external camera to the seat back display.
Adding galleys and toilets where there is no current provision could prove expensive, although if the lowest deck seats were only offered to able bodied passengers, they could use the upstairs toilets.
Lots of other stuff to be considered, but if it could give the equivalent of an extra 737's worth of passengers per flight on busy short haul routes then it could be worth the expense.