Woo, right up my ball of wax.
I'd argue that one of the biggest factors that significantly impacted the construction of the Cincinnati Subway (and it's original funding in the form of Modification H) was the concurrence of both the United States joining the First World War, the declaration of Ordinance 96-1917 illegal, and then finally the passage of the 18th Amendment. Ordinance 96-1917 made the Cincinnati Street Railway the leaser of the Rapid Transit Loop, and the Ohio Supreme Court ruled that Ordinance 96-1917 was invalid for whatever reason (it does not mention in the book) despite New York City having a similar setup. The rampant inflation as a result of the First World War significantly impacted the available money to the project, and the passage of the 18th Amendment in turn reduced the available tax base that Cincinnati relied upon, which thus tied into the issues of it hitting it's debt limit.
Hypothetically speaking, you could have had Murray Seasongood (who led the Charter Movement in reforming the Cincinnati city government and remove the preexisting local Republican machine) to have it succeed in some fashion along the West Side, but other issues emerged forcing it to be ended. In such a case, I'd probably place the chances under Seasongood for a completion of it as it existed, with it being extended to Fountain Avenue (which was where 47% of all origins and destinations for the trips were) as the terminus. This
map would have nearly been the final result as it stood, with it extending somewhat to the south in Cincinnati for the terminus at Fountain Avenue.
One of the more interesting components of the Cincinnati Subway was that of Race Street Station, with it's fully intended five track operation. This was the case because of the Cincinnati Subway's operation of interurbans alongside the regular 'subway cars', where it was planned that the outer two tracks would be fully served by the full length cars (considering that the 'inner' two tracks would have had doors opening against columns), and the inner two tracks along with the pair of middle tracks (each terminating half way through) being operated by interurbans and then being turned at wyes on either side. How this would have faired with the eventual collapse of the interurbans would have been interesting.
The matter of the Cincinnati Subway and I-75 would be interesting, considering that along a fair amount of the route that the Cincinnati Subway was planned to run along was eventually supplanted by I-75. I could see some changes along I-75 as a result of how it winds downtown, because of the existing infrastructure though, although I'm not sure where.
I'm not sure in term of the effects of the development on Cincinnati throughout the rest of the 20th Century though. It might have encouraged the development of CVG being in Blue Ash rather than in Boone County, Kentucky (although you'd likely need a reduction of the supermajority laws from 65% to 55% much earlier, or have it as 55% originally rather than the 65%).