Don't see it happening. Swappable storage (be it for upgrades or transient media) requires several times the space of a fixed on-board IC. Consider the volume increase: take the bare essence of a Micro-SD device, then wrapped in user-resistant protective casing, add connector, slip that into an SD adapter, which then goes into a environment-resisting slot casing including a slight but relevant air gap, which uses SD-sized connector receptacle, in turn soldered onto the main board with a total footprint much larger than the actual storage IC you're trying to connect - for a device as compact as the iPad, that's a LOT of wasted volume, a lot of "low benefit per cubic millimeter". Yes, a slot would be nice - then again, we could stuff several 64GB flash chips in that space, or another processor, or more battery, giving much more bang for the buck.