I do not expect any Broadwell minis. Skylake is just behind and there won't be two updates to the Mini in under 2 years. Expect the next Mini, or 'Mac Air' to bring some consistency the the names, to be exactly a Macbook Air without the display in a tinier enclosure:
SSD (PCIe) only
Thunderbolt 3
DDR4 (soldered)
USB 4
2-core
4k@60Hz-capable
No discrete GPU
I would expect a price drop on the new quad-core Mac Pro to help fill the middle ground but neither the Pro nor the Mini are even the focus of Apple's Mac development, let alone their overall business.
Unless Apple want to go the full hog to demonstrate that they have the greenest computer on the planet, which is a distinct possibility, I would expect continued neglect of the Mini line. There is a business case for a cheap Mac but I get the impression the concept is almost beneath Apple.