Another fun one: a bridge to replace the Mukilteo-Clinton ferry, as studied in 1968 (and found to be infeasible even with tolling).
The most "feasible" route would go over Hat Island to Everett in order to avoid a 620-foot depth and instead only need to deal with a 500-foot drop. All at a cost of $160 million in 1968 dollars.

Since the Hat Island alternative crossed two channels, it would be likely that the portion over the deep/navigable channel would likely have included a suspension span across the depths (likely as lengthy as the Golden Gate or Verrazano ones to clear the 500'+ deep channel); the other longer bridge to the east could have been done as a simple viaduct with one raised section to clear the deeper portion for navigability purposes. Nevertheless, it's better that no bridge was built; Whidbey certainly doesn't need any more casual traffic -- and the idea of a bridge today wouldn't likely get out of the starting blocks -- although the shorter high-clearance bridge might have been done today with a cable-stay span, a type that didn't see favor until around 1980. But since the east end of the bridge would have been in Everett, there's a chance that it may have been considered to be a US 2 extension that would utilize the Port Townsend ferry to get to a US 101 terminus.
I would hardly say it is "better" that no bridge was built, it would provide a vastly superior routing to what is currently available. Building it today would have course fall prey to the usual problems with NIMBYs, special interest groups, activists, etc.
I totally understand why they wanted to build the bridges. I could also get behind some of those ideas, back in the day. However, today, with the Naval Base in Everett, there are few viable options, and the most logical one actually goes from Harbour Pointe to Possession Pt.
That being said, I like being isolated from the crap that goes on in Everett, I just wish I could convince my fellow Islanders, that we need to try to be more self sufficient, with adequate housing, employment, industry, and medical services. And maybe a grocery store that is open until midnight, like they were 30 years ago.
Of course, on the north end, an alternate to Deception Pass is sorely needed.