Disregarding new games like acdc etc that may still require coding updates, are there any older games that have obvious incomplete rule sets. Also of those, are there hobbyist out there capable and allowed to modify and attempt to complete such games. Is there a black market for roms that have improved rule sets for any games?

Older Games - The pinball industry was more interested in catering for the operator. Rules were simple, and didn't need refinement. Having said that, the Gottlieb SYS80 /a /b had some very complex rulesets for their time. But none of them were considered "unfinished". Just very difficult to understand (during their operational cycle) - which may have been part of the strategy for players to continue inserting coins to learn the game. I remember pumping a lot of money in GTB Spirit and I still didn't understand it until recently. Code in old machines really tested the designers, or the designers wanted to push the coders. Either way, old code was never really developed until Pascal came along with his series of boards which added new features - but didn't complete "unfinished code" - more or less "enhanced" the code.