The most intriguing feature along Route 81 is probably the 5-ramp interchange with Route 9 (shown) with direct access to 9 north from 81 north.
Freeway plan -- fictional
The first Route 81 freeway plan I found exists in fiction: the plot of T. E. D. Klein's short story "Petey" hinges on a state plan for upgrading Route 81.
(In some editions, including the "Dark Gods" collection, the highway is instead Route 501; but Route 81 is more appropriate for the story's Clinton or Killingworth location. I do not know which number was used first. Did 81 replace 501 for verisimilitude; or did 501 replace 81 to minimize any potential notoriety for the fictional "highway department" that some would read as ConnDOT? SR 501 is the once-planned I-84 spur into West Hartford Center.)
Freeway plan -- real
In 1969, the Mid-State Regional Planning Association proposed a freeway linking Route 9 to Hammonasset Beach State Park in Madison. This is similar to an earlier Route 79 freeway plan that would have extended to Route 66 in Middlefield instead. The proposal was likely never advanced and is not active now.
The preliminary plan proposes the construction of a connecting highway, roughly parallel to Route 81, which would connect to the Connecticut Turnpike and Hammonasset Beach State Park in Madison. This Hammonasset Connector is an implied part of the Tri-State Transportation Commission's plan which covers New Haven County.