I've been reading through some of the old codex's and chapter approved books and thinking "wow, why can't these rules still exist?". Then I realized they could! A little bit of work and they could easily be updated to 6th edition. A good example of a rule set is in the Codex: Armageddon I think its called? GW has a set of wargear options and alternative FOC options for the Salamanders chapter. Their veterans can all take artificer armour and/or drake scale cloaks, even the sergeants! They could take ceramite armour for all of their vehicles (25pts for Land Raiders /10pts for everything else) which gives them immunity to the melta special rule. Small but crazy things like that give the army a distinct feel that sets them apart from the vanilla
So while I'm still very behind on my project to make league rules for the Arena of Death. (blame play testing) I think a next cool project would be to find these different rule sets that made the armies and chapters unique and update them to 6th ed in a internet approved rule supplement. Not being official it could only be used in your friends basement but if it grew as an idea and took off GW might finally notice and make an official book with rules for chapters and war-bands again. It's a long shot but if anything the idea of making money off of a rule supplement that a lot of people would actually find relevant might perk their interest.
Edit: Another good example of chapter/legion specific rules is anything in Index Astartes. Still no reason this couldn't exist in 6th Edition.