I think everything hinges on whether they keep the form factor the same or not.
If they keep the form factor the same, it can almost certainly maintain support for 2.5" drives and thus the entry level will likely start with a 500GB spinner with SSD BTO options. Similarly, the same form factor would almost guarantee the same RAM compartment on the bottom and thus user upgradable RAM. And, if they stick with the same form factor, it might offer enough cooling to support desktop Haswell CPUs as in the iMac.
If they redesign the Mac Mini, they can reduce the size significantly by moving to a single PCIe SSD blade used in the MacBooks. If they do this, 128GB SSD would be the entry level config. A redesign could very well jeopardize user upgradable RAM. A new slimmer form factor would also probably mean mobile Haswell CPUs similar to the MacBook line up. The whole thing could probably be half the height of the current Mac Mini if they wanted it to be with the PSU perhaps being the limiting factor in size reduction.
Bottom line: I believe a lot hinges on form factor.
For ports/RAM:
Thunderbolt 2 is unlikely to make it into this budget oriented computer. I would expect TB1 only... with dual ports (which are standard on Intel's controller). Firewire would almost certainly disappear. Ethernet and USB3 will remain. The only question mark in my mind is HDMI... a redesign could eliminate native HDMI requiring a TB-HDMI adapter for those of us using them as HTPCs.
RAM could start at 4GB, if only to meet a price point, although 8GB seems to be the standard across most of the Mac line now.
Last but not least, I really wish they offered the Mac Mini in BLACK! It currently looks like an odd duck in my HTPC setup.
Your analysis seems accurate, but fells along memory.
I must to said, there is no chance to see ram slots, since Apple will move the mini to s new logic board (from ivi bridge to Haswell), apple move to soldered ram, saves cost and service calls, only high level system will keep upgradeable ram (as the iMac 27 and the Mac Pro), you may not agree but actually few Mac users upgrade their Ram, and is mostly due they save purchasing underpowered Mac, and as soon they put they hands on, swap ram modules for beefier ones (I used to do that), Apple it's aware this, and aware on the solution (soldered ram), they may make few DIYers cry but actually doesn't affect sales, but saves cost and users use to order reasonable spected Mac.
So next mini has little chance to get ram slots, while maybe cheaper as return.
No chance for a new form factor if they simple quietly update the mini, this is sad, a new form factor will be announced at am media event at least, it's what deserves.
A Haswell update may offer same cpu we see a the iMac or the rMBP, unlikely they remove the spinner, they didn't with the cheaper iMac less likely to do that on the mini.
About Thunderbolt, rMBP comes with TB2, coz it's part of the Haswell chipset and the fisical connector it's the same, to offer the Mac mini Apple will require a sort of downgraded chipset, unlikely.
Hdmi port has no risk, as the rMBP users cry for an extra usb3 instead hdmi Apple kept the hdmi port.