Keep the design in files with an open format, editable with freely-available design software. (v2.0 moved the project to KiCad, which has gotten considerably more traction than gEDA.)
Replace through-hole components with surface-mount components to the extent that it's possible to do so. (It should still be hand- solderable, though an inexpensive hot-air rework station of the "858D" type would be better for SMD parts than a soldering iron.)
Enable future improvements to the design by anybody interested in doing so (item 1 helps considerably with this).

Discussion in comp.sys.apple2 has already led to one change: provision has been made to clock the 6551 with either a crystal (and, if needed, an extra capacitor) or the GPIO4 line from the Raspberry Pi. v2.1 adds the ability to use a 3225-footprint SMD oscillator.

Also, I had originally planned on using the WDC W65C51N in the LQFP-32 package, but there's a bug in the W65C51N that indicates it might be a better idea to use new-old-stock 6551s from other suppliers.

That is why the 6551 is still using the old DIP-28 footprint.

My initial design worked, but wasn't without issues. Most importantly, at least with the Raspberry Pi Model B, there was interference between the 6551 on the adapter and the electrolytic capacitor next to the micro-USB jack on the Raspberry Pi. This design places nothing on the board under the Raspberry Pi that could interfere with it.

Also, while it doesn't use any of the extra pins, it now has the 40-pin GPIO connector used by current Raspberry Pi models. Original models (with the 26-pin header) will still work if you build the board with a 26-pin socket; a 40-pin socket will interfere with the composite video output.

This design depends on symbols and footprints I have modified and/or created. They're included as a submodule from my library.

You can find this project on Hackaday.