There's nothing with the 21-70 format that will allow faster charging. The format is likely a obstacle to faster charging, as the delta T between the center of the cell and the surface of the cell increases with size. If there is faster charging in the very near future, it will be driven by improved chemistry. And this chemistry could be incorporated into the 18650s without much hassle.
After thinking a bit more about what I think will happen next with the Model S/X battery packs, here's my thinking:
1. The 90 kWh pack is updated to the new module design, and goes from 16 to 14 modules. (Soonish.)
2. The 90 and 100 kWh packs get cells with the newest chemistry, upping the capacities by ~5% and potentially improving charging power. (This year.)
3. The current 75 kWh pack is eliminated, replaced by a lightly modified top spec 21-70-based Model 3 pack, which is also around 75 kWh. (This year.)
4. The 90 kWh and 100 kWh packs are completely redesigned, the 75 kWh pack uses 8 modules like before, while the ~95 kWh pack uses 10 modules and the ~110 kWh pack uses 12 modules. (2018-ish.)
This allows Tesla to first phase out the outdated 18650 module design, then they phase out 18650s entirely, going to a single module design across all vehicles.