Given the above, what would you have the Northeast Extension of the PA Turnpike designated as?
Really it was too long for an auxiliary route. But an I-x80 or I-x78 could have worked.
Given that the Northeast Extension's northern terminus is at I-81; one could've used I-x81.
As far as length; both pieces of I-476 (Blue Route & Northeast Extension) total roughly 131 miles; 111 of those miles being the NE Extension. So I-476, in total, is longer than I-495 in MA by about 11 miles.
I thought about an I-x81, but decided not to given the low-grade connection to I-81.
IIRC, a remedy for that connection is now in the works.
Your suggested I-x78 wouldn't work because although the two highway cross each other, they don't directly interchange w/one another. One needs to use US 22 and possibly PA 309 for connections. Your suggested I-x80 alternative has an issue regarding I-476's connection w/I-80 involves going through a signalized intersection w/PA 940.
How about a mainline route like I-67? Wasn't that discussed for the western part of the state?
Over the years/decades, there's been three different proposals out there for I-67; not one of them is in or involves PA. The western PA that's been touted in this forum is a fictional one.
That said, and there probably already a fictional thread on such, a more desirable 2di candidate for the NE Extension would be to have it be a part of an overall northern extension of I-85. Yes, such would be concurrent w/I-95 in many areas (utilizing only current Interstates along the way) but there's enough parallel or semi-parallel highways in key regions where the two could be separated for some distances.