Great thread!
I too noticed Shanny when from the Traditional WCO to a zone blocking run scheme with various I sets...although, Jim Fassel (former Bill Walsh disciple at Stanford) was sort of toying with that same stuff as OC in Denver the year before Mike was hired. Makes me wonder if Shanny installed it all himself, or kept bits and pieces that Fassel put in place (and how much).
Gary Kubiak (former 49er QB) went over with him as well, and could have had a say in some of the foundation as well....
When Kubiak went to Houston, you could see he basically took Shanny's playbook with him and added some more Ace/Singleback sets.
Interestingly, Mike Heimerginger (RIP) who was a college roommate of Shanahan's, utilized the exact same formations during his time as Titans OC except he went to a power/man run blocking scheme.
Mike Holmgren added the spread look with a lot of flexing of the TE like we use now (Mark Chmura was good at this as well as Sterling Sharpe in the slot). It really hurt teams in the late 90s because most defenses were still running a traditional FS/SS scheme, with the SS being more like an extra LB. Putting the TE in the slot or out wide forced teams to defend in awkward positions whenever they played the Packers. It was a subtle but deadly wrinkle.
Mooch took it with him to SF and it added some more productivity towards the end of Brent Jones' career.
I could go on....I love the WCO