Angry Video Game Nerd did videos about adding the Sega CD and 32X to the Sega Genesis and what a pain it is, with all the extra cables and wall warts. And there are a bunch of videos about how to mod your Genesis to support NTSC and PAL, ad better picture and sound outputs, and even overclock the system. All this would have been in the aborted Sega Neptune.
I'd like to see you build a best-of-breed Neptune-like out of all the above Genesis hacks and add-ons. Bonus if it can also play SMS games!
https://www.youtube.com/watch?v=g2eH3vYbdGo
https://www.youtube.com/watch?v=VvR_3OTxs8A