Too small. It needs to be something that can compete with other pro workstations - HP Z, Dell Precision, Lenvo ThinkStation, etc. That means support for multiple GPU's, dual CPU support and at least 512GB RAM. There needs to be room for a decent amount of internal storage 6+ drives, but also standard connectivity to external arrays - read that as SAS, FC, 10Gig-E. Thunderbolt can be there as an option, but FC still wins in the corporate world. Options, options, options. Not everyone is the same or has the same requirements. Apple needs to build a machine that is flexible enough to encompass all of these options. It won't be cheap, but the intended market wants cost-effective, not cheap.
There will be a massive gap between the Mini and the Mac Pro if Apple release such a machine, even more than there is now. A modular design like the one here wouldn't work for those guys either though as a gaming GPU would probably be too much for the PSU. So it could be that the Mac Pro has to start as a single socket design with a dual socket option - same chassis and everything. Could even start the single socket design with i7 like the X99 platform.