I Like the stacking idea, where you would have pictures or links on the waymark page of say the 6 closest waymarks. When you go to a waymark page, if you click on search for nearest waymarks you will generally pull up any cross posts for the same location, but it would be nice to see them on the wamark page instead of opening another one.
I think the idea of a gallery for each waymarker where you could pull pictures from over and over would be great. It would have to cut down on storage since you would only upload once instead of each time you used a picture.
I am definately opposed to limiting waymarks to only one catergory or anything even close to that concept. Cross posting is one of the things I like best about the game. As Silverquill said, if I'm only interested in NRHP buildings and one of those is only listed as a WPA project then I would miss it. I've had it happen where I went to waymark a local lighthouse in the lighthouse category and found that it was also NRHP and Relocated all the way from Deleware to FL. I posted in all three categories because I thought it was great info and wanted to share it. Besides...I love Lucky 7's, how would limiting affect that category? What would happen to Lucy? http://www.waymarking.com/waymarks/WM4A5Z_Lucky_7_Lucy_the_NRHP_Elephant_Margate_City_NJ My daughter studied her in school and we pulled her up on waymarking. She learned more on waymarking then she did in her 10th Grade classroom and was able to take that info back to her classmates. I see great educational potential in this game if it's played toward that goal. This waymark is a great example of WHY to crosspost. Someday I'm going to NJ just to see HER. 
Anyway...What about a variable that said " I have also listed this in _____ catergories?" Or a drop down where you could choose multiple category listings by holding the control key. I'm tech impaired so I don't know how any of it would work.