The original RR didn't have most of those types of cars. While they might be fun to have, they don't fit into the mission of the museum.
Remember, Matt, this isn't our own personal train layout. We are trying to recreate something as it existed 100 years ago. Now the WW&F had plenty of interesting rolling stock. We've also talked about new replica cars on multiple occasions. (I *think* I converted an old thread about this last night. If not, it's coming.) Most of that was BEFORE we acquired excursion 103, coach 8, and what became flatcar 126.
However, the reality is our yard is full now, and we have plenty of other things that need to be done first: Bathrooms, Roundhouse, Visitor's Center, etc. Once we have gotten to the point in the master plan where we have constructed the car house, then we can turn our attention to ways to fill it.

Plus, I think it would be more fun building #11 before we add any rolling stock. I'd also like to see coach 8 outfitted with proper seats.
So much to do, but it is good to dream.