Reviving this, I can see elven wstone being used a lot. I used to not use any other races wagons until I figured that rather then leaving units behind or sacrificing them because they're too slow, I could pack the m into a wagon and rush them to the enemies base. It also provides protection for units inside. As for that catapult thing technically that could work. Think of it like this, Humans take the catapult or trebuchet apart and stick it into the wagon, unlike building it were you need to gather the materials, they have it all together and just have to put it together. Most of the work people do from scratch is gathering resources for the project. If you have all the material their with the tools you can put it together fairly quickly. Giants I agree with you on this and now that ents are going to be turned into siege towers basically they don't need to be put into this.
Remember to check the unit ready list for AOF to see for any changes.