This was back in the early 90's, so it was awhile ago. I ran a 50's-early 90's AC format with an equal emphasis on 60's, 70's and 80's with 50's and 90's (then recurrents and currents) as spice. I played one current and one recurrent most hours. For the most part, it was a mid to down tempo format. I transitioned from Standards, so there were some leftover artists that lingered, like Johnny Mathis, Jack Jones, Patti Page, etc. Not too many of those though.
The bulk of the format was the big artists you hear on Standards today and AC from the 80's & 90's, with cores like Billy Joel, Barbara Streisand, Elton John, the Beatles, Gloria Estefan, Rod Stewart, Fleetwood Mac, Simon & Garfunkel, Linda Ronstadt, Barry Manilow, America, Bette Midler, Neil Diamond etc. There were a few country crossover artists like Ronnie Milsap, Willie Nelson and Restless Heart, but not a lot. The station was not in an area where country music was dominant.
My biggest requirement for songs is that they needed to be a big Top 40 pop hit, not just an AC hit, country hit, or an oldie that mid-charted. Every song and/or artist had to be recognizable to the 25-54 demo (at that time.) No stiffs.
It sounds like an updated version of this format, maybe concentrating on the 70's through the 90's with a few selected titles from the 2000's would work, but not overdoing it on the newer material. It would depend on how young the Oldies station is. You'd probably want to stay away from the 60's and be selective on the 70's, maybe just playing the slower stuff and the mega hits. Is the Hot AC playing a lot of 80's? Is the Oldies station? It could be a hole that you can fill, while keeping the tempo down. Sounds like an interesting opportunity for a softer, gold based AC station!