At this point a redesign seems somewhat likely, probably ditching the spare drive bay in favour of an even smaller and more compact design, possibly using an SSD blade to provide Fusion Drive options within the smaller space.
Spec wise though I'm not sure what you consider huge. The Mac Mini may get a second Thunderbolt port, but if Apple do go smaller I'd say it's more likely they'll just drop FireWire 800 and leave a single Thunderbolt port. Not sure how likely it is to upgrade to Thunderbolt 2 or not; Apple do prefer to go for consistent ports specs across the range, so it's possible, will just be a bit weird when the iMacs don't have it yet.
Intel's Haswell processors aren't all that much faster in CPU performance compared to what the Mac Minis get right now, the main benefit is the much better integrated GPUs for both gaming etc. but also for OpenCL performance (which may become a bigger deal in future, but also may not, it's hard to judge). In fact the Iris GPU should be pretty decent for casual games or older games from a few years ago. The most interesting CPU option will be Iris Pro (if it's even offered) as it has a Crystalwell 128mb cache which will make the GPU perform more like a discrete GPU, but it can also accelerate the CPU, or any combination of the two. I wouldn't expect that except maybe on a built to order quad-core option and/or the server model (if they even continue to offer it, personally I doubt it). I think it also boosts performance of virtual machines by a small amount, but not loads, just in case you run any.
Otherwise I'm not sure what else to expect; MacBook Airs still have 4gb of RAM as standard, so I doubt the Mac Mini will make the jump to 8gb as the majority of users don't need that much anyway (especially now that Mavericks has added RAM compression). Might get some more HDD options, but again the majority of users don't need much either.
So yeah, unless you're really interested in OpenCL and/or gaming then I doubt the Mac Mini will be a "huge" spec-bump, and if you've always looked at the Mac Mini and thought "damn that's one bulky computer" then the redesign might be of interest, but otherwise I expect it'll be mostly incremental.
I'm on an early 2008 Mac Pro so I'm definitely waiting for the next Mac Mini, as even the current quad core Mac Minis outperform my dual CPUs, and the Haswell graphics improvements mean the new ones should also outperform my GeForce 8800 GT; not by a massive amount, but enough that it could still be a bit of an upgrade, especially when you consider much faster RAM, Thunderbolt and USB3.