These are some cities/towns in Northern Italy that I have visited and enjoyed and that you have not yet visited: Ferrara, Trento, Sirmione, Vittorio Veneto, Aquilea. Ferrara and Trento are smaller cities but very appealing, each with a castle and beautiful main piazza/ cathedral area. Sirmione, on Lake Garda, is very picturesque. Vittorio Veneto's historical old town is charming and Aquilea is famous for its Roman ruins.
These are places I have not yet visited and you have not mentioned, but are on my Northern Italy radar and wish list for future consideration: Mantova, Trieste, Chioggia, Bergamo, Brescia, Camogli, Tremosine sul Garda, Bassano del Grappa and Vipiteno. A friend of mine who lives in northern Italy also recommended the small towns of Monselice and Cittadella, one north and one south of Padova.
It would be very easy to add Ferrara and/or Mantova if you are in the Bologna/Parma/Modena area. I liked Ferrara very much. It was easy to walk to all of the key sites and it was not full of throngs of tourists. Same can be said of Trento, plus you have the beautiful mountain scenery surrounding you.
After having made several trips to northern and central Italy, my next priority is to explore the south.