The PC Engine (TurboGrafx-16) had many, many hardware revisions...
http://www.ausretrogamer.com/wp-content/uploads/2012/03/PC-Engine-Genealogy.jpg
...but never an ultimate one, that had every feature.
The build idea is to mod a PC Engine to include every feature of every revision, in one unit. Features...
Level 1 (Hecky dory)...
1) Plays both JP (Hu Cards) and US (TurboChip) games (region free mod)
2) Plays CD games (already region free)
4) Plays games that requires the Arcade Card (Neo-Geo fighting game ports, mostly) by building in the Arcade Card and have a switch for it
5) Has the 5-player adapter built in (normally only one controller port; not even two)
Level 2 (Hecktastic)...
6) Plays SuperGrafx games (only 7 unique games)
7) RGB, component, and/or S-Video out mod
Level 3 (Heckinator)...
8) Slot-loading CD drive
Level 4, They've Gone to Plaid (Heckdiculous)...
9) Plays LD-ROM LaserActive games
The PC Engine was a great system and, because of its extensive lineage, it's begging for such a mod. Call it the Turbo Trio RX Super CD-ROM³