I saw a 2.3 quad-core i7 Mac mini with 8 gigs RAM got within 10 points of the new quad-core Xeon Mac Pro on Geekbench 3 today. Given that, it doesn't need much, and Apple probably knows this better than anybody. Anyway, here's what I'd like to see:
Haswell processors for the graphics only. Leave the rest (and cost) as is.
I agree with Peter: trade in the Firewire port for a second Thunderbolt port and raise it to Thunderbolt 2.
WiFi up to ac.
Leave the rest as is. An internal PCIe plus SATA could increase the price. Let's not go there. The current Mac mini Fusion drive runs just over 500 R/W on Blackmagic, which is plenty.