I'm hoping they compromise and use 7200rpm HDDs and offer PCIe SSDs/Fusion as a BTO option. I don't think it would be that difficult if they're coming out with a new motherboard for Haswell anyway if they managed to fit a PCIe connector on the motherboard and use some kind of break-out lead/mounting system where the bottom HDD currently goes. That way they can still offer a HDD to keep costs down, still offer dual HDDs for a server model and yet offer PCIe SSDs like the rest of the Mac range can have either natively or as a BTO option.
I also hope they start using some of the more recent 1.5 or even 2Tb 2.5" 9.5mm HDDs. If they offered either a 2 x 2Tb server version or a 2Tb Fusion with PCIe SSDs instead of the existing 6Gb/s SSDs it would be an excellent upgrade even if the rest of the system remains the same accept for updating to Iris GPUs, Haswell CPUs and Thunderbolt2. I wouldn't be surprised if it drops Firewire 800 and has 2 x Thunderbolt2 ports.