I just added an SSD to my 2011 Mac Mini (base model). It's running Sierra just fine, and performs admirably. Other than not having a GPU powerful enough to drive a high-res monitor, it isn't lacking in any features present in the new Minis (or the iMacs really). We can complain about the stagnation of the Mini, but the only likely changes would be 1. form-factor (smaller, new ports), and 2. more powerful hardware. Altering the form factor isn't as major a concern even with the Mac Mini as it is. In the case of the latter, the CPU situation is stagnation on the part of Intel and AMD, and has little to do with Apple in many respects. The nature of computing is moving towards silicon dedicated to hardware acceleration of specific tasks--as opposed to GPGPU--and usage of specific extensions. The uptake of any of them has been fairly slow across the PC market, and it's not likely to improve in the near future. Cramming more cores in the Mini might be nice (with improved cooling solutions in particular!), but it was never designed to be more than a standard desktop or rival the Mac Pro.
If they can update to the latest processors and continue to have an Iris option, add M.2 storage, and update the ports, I'm not sure what else they need to address for now.