This presents the issue of easily making quantum stockpiles, which toady wants to address very soon anyway along with the hauling changes.
I wonder if some kind of fluid dynamic might be a good fix for that. Say a tile can hold 10 items, so once it's full additional items will spill out to adjacent open tiles. This would be a good way to properly simulate big mounds of treasure in a hoard, or a mounting stack of coal being brought to your smithies via mine carts.
If items have a size or volume attribute, then each tile could have a total capacity which will vary in quantity based on what the items are. A few bins might stack, whereas a few dozen loose crafts will probably spill all over the place. Clever players could use this as a sort of warehousing system, with increased economy of space in exchange for the extra work of manually stacking items.
Full-capacity tiles shouldn't completely block movement-- that would be hell on pathfinding, not to mention introduce intimidating complexity for newbies-- but force dwarves to crawl through the space, which will introduce inefficiency and slow down work if not accounted for. This would encourage players to set up proper storage spaces instead of any old place.
Once items have a liquid-type flow mechanic, it makes sense to incorporate item volume and liquid volume together. Items will displace liquid, or be displaced (float) if they have low weight-to-volume ratio. There might already be something like this in the new splash code.
It all seems rather complex, but I think it would fit with dwarf fortress pretty well. It would open up activities like filling a lake or a well (or a caldera) with rocks in order to raise the fluid level. Clever folks could probably set up custom pumps and liquid cannons this way, and item stacking rules have been a long time coming.