I don't see what the point of USB or networking would be; a NuBus Mac can use an Ethernet card perfectly well, and why would you want to use a USB keyboard or mouse with a classic Mac.
Really the big interfacing issue with a classic Mac is video, because many modern displays can no longer handle their sync frequencies. Either you need an older CRT (and to maintain it) or an older CFL-backed LCD (and to maintain it) that can hopefully show an image at a non-native size without upscaling.
With a custom digital video card and DVI/HDMI output you can actually hook up a modern display, just as with the card the OP created for Amiga. But unlike with Amiga, the classic Mac always used a straightforward framebuffer, so there's no software support issue.
There are other (non-DIY) USB-to-ADB projects out there taking shape, too.
But I'm also hoping someone starts making Nubus cards for VGA, USB mass storage, etc. It would just take an FPGA or perhaps even an overclocked teensy.
Not much; Zorro is basically the 68000 bus protocol, while NuBus is more general. But it's just a multiplexed address/data bus with a selectable 8/16/32-bit l word size for data transfers, it can be implemented with a simple state machine.
All you really need are the level shifting buffers just as with the Zorro interface, and enough I/O for the 50-some NuBus signals. That breaks down to 32 address/data lines, a few NuBus control lines, and some lines to control the level shifting buffers.
Really the big interfacing issue with a classic Mac is video, because many modern displays can no longer handle their sync frequencies. Either you need an older CRT (and to maintain it) or an older CFL-backed LCD (and to maintain it) that can hopefully show an image at a non-native size without upscaling.
With a custom digital video card and DVI/HDMI output you can actually hook up a modern display, just as with the card the OP created for Amiga. But unlike with Amiga, the classic Mac always used a straightforward framebuffer, so there's no software support issue.