Mac nano
They got rid of the optical drive but kept the same form factor because it made sense at the time. There was no need to design a new case while the Mac mini was in mid-life cycle.
But now, if they get rid of the 2.5" drive and go with a PCIe flash drive as they already did in new Macs, they can probably squeeze everything into a much smaller case. They need to re-use their custom components in as many computers as possible to lower the overall price. I see the Mac nano with 128GB flash storage for the entry model to keep costs down.
For the RAM, either they keep two slots for a maximum of 16GB which is still plenty for the entry-level Mac, or they go with on-board RAM like the MacBook Air in order to cut costs and size requirements. If that happens, then the options are probably going to be 8GB or 16GB.
Don't forget that the current generation only has Intel HD Graphics 4000. Since it's going to be Haswell CPUs, it means Iris/Iris Pro graphics so it's an upgrade from HD4000. No dedicated GPU means a smaller motherboard, a lower cost and a lower TDP too.
A smaller aluminium case would require less time to manufacture and would be more compact for shipping and inventory. Don't forget we are in the Tim Era now.
All that for the same price as before.