Good point about what kind of trips the sleeping cars are designed for. The longer the trip the bigger the demand for more hotel-like amenities. And older passengers may have a greater desire for ensuite facilities or a say a lower berth two adults can share. Personally I think having all roomette (VII style) and all bedroom cars (in lieu of or in addition cars w/ both) is worthwhile. ADA requirements are stumbling block to a all roomette cars. Maybe the solution is to operate the sleepers in married pairs like some of the Amtrak Midwest coaches? Would that count as a single large sleeping car? If not what if the all bedroom car had 2 ADA rooms?
A Superliner would be easier to add ADA rooms to a Superliner since if the whole upper level consists of Bedrooms instead of Roomettes, the lower level probably could be converted to all ADA rooms, but that doesn't take away from each car needing at least 1 ADA spot. An all Roomette/Section single level car could be done, the question is would a Section count as a seat or as a room. If its a room, then you would need an ADA room. This would still mean 32 people could ride in this car outside of the ADA room. This is the number of passengers the first incarnation of the Slumber Coach could hold and the maximum a Tourist Sleeper could hold.
Cars in the past were married together. The Southern Pacific did so for its Daylight equipment and even had a Triple Section Diner. The caveat I would say to that is I doubt disability advocates would allow a married pair car, one with an ADA room and one in an all Roomette/Section arrangement. The reason they would go after this is why should able bodied people get a discount sleeper and disabled people don't? I would agree with them on this. This problem is easier to solve if a Section is categorized as a seat instead of a room or if they used lie flat seats instead. But this is all a hypothetical until Amtrak is run by people who's first 2 requirements aren't political connections and a recent head injury.
I would doubt they could put 2 ADA rooms in 1 single level car without adding a second door. I've heard from others that navigating the single level trains is next to impossible since the halls aren't that wide.
It would make sense for Amtrak to have some version of a Tourist Sleeper/Slumber Coach, Deluxe Sleepers and some of the existing ones to make up the difference in capacity. The problem is Amtrak leadership doesn't really care to make things work well, most of them are there because of political connections which is a poor way to run anything.