What if you had the option to designate a foundry project slot(s) to storage space for personal assets: Costumes, object groups, maps, customized mobs, ect which could be freely exported and imported from here, allowing you to repeatedly access them without having to re-create them from scratch.
Each slot so designated would give you X mb of space to dedicate to this storage area and the author could also designate items as "public" which would allow any foundry author to go poking around for something they might find useful.
I don't think you can actually connect them in a parent/group sense. You pretty much have to just position them individually so they appear connected.
That said, the ability to group and/or parent assets to one another is another thing that would be awesome to have in general. It would make building and tweaking maps soooooooooooo much more efficient and less of a PITA.
There are some stock maps that come with things already put in place. Also some of the assets do look like they are connected when lined up properly, but are actually just positioned close together by an author so that they appear to line up.