Personally, I think the form factor of the Mac Mini is going to stay the same for this next release. There has been zero leaks of information or photos to suggest a different form factor.
I do expect the big changes for the Haswell Minis to be:
1x PCIe SSD (Same as new Macbook Air)
Bluetooth 4.0
AC Wifi
I think they'll redesign the form of the mac mini over the next 12 months or whenever they can get hold of the Broadwell chips from Intel as Broadwell will necessitate a new motherboard.
While serving a totally different market from the Mini the new Mac Pro is likely the only new design we'll see for OSX products this year. Apple wouldn't blow is its aesthetic load twice in the space of a month with a new design for the Mini and a new design Mac Pro.
This year the Mac Pro is the priority and next year I would imagine they can put the bells and whistles on the Mac Mini (Thunderbolt 2 etc...) but the Mac Mini is still a very capable machine for most peoples uses.
I would imagine a redisign of the Mac Mini would make it even smaller and lighter and potentially making it the home hub machine and for it being part of

and their plans to compete in the living room.