Most of isn't "retro-next" computing. There are a _very_ small number of projects of that kind, most notably the X16, the Mega65, and the Spectrum Next. The Maximites might fall into that, as might the MiST and MiSTer boards, but that's debatable. What _most_ people do are extensions to existing hardware (like the Amiga Vampire), replacements for failing chips (which might also include extra features because there's space still left on the FPGA), &c. Arguably, I wouldn't include the Mega65 in the list because it's meant to be more or less what was intended to be released as the Commodore 65, which basically leaves the X16 and Spectrum Next. The X16 and Spectrum Next are meant to be
idealised forms of the respective machines they're based on, with the Next maintaining compatibility.
What most people build are things like Z80-based SBCs that behave like the CP/M machines of the past, but can take advantage of things like CompactFlash and higher clock speeds, but are essentially period accurate otherwise. Personally, I'd love to build a Jupiter Ace with all the random glue logic reduced down to a PLD, expanded enough to support an ~80-column display.