You have obviously developed an addiction to Italy, which is actually more common than people might think, and for which there is no cure.
In my opinion you have the right approach to travel, in the sense that in each trip you focus on a reasonably limited geographical area approximately the size of a small New England state like Massachusetts. Each Italian region has a lot to offer to the visitor to keep them busy for multiple weeks, so that is a good thing. That approach also saves a lot of ground transportation money, because you move only for short distances.
To continue on that same pattern, I suggest you divide Italy into its regions or macro regions.
I will start with NORTHERN ITALY
TRIVENETO (the North East): this macro region comprises 3 administrative regions (Veneto, Friuli Venezia Giulia, Trentino Alto Adige). This area includes cities like Venice, Padua, Verona, Vicenza, Udine, Trieste, to name a few. It also includes mountain areas like the Dolomites, lakes like lake Garda, Roman ruins like Aquileia, and much more. This macroregion alone will keep you busy for a month.
The NORTH WEST: LOMBARDY, PIEMONTE AND VAL D’AOSTA: this area is the north west of Italy. This macro region comprises 3 administrative regions (Piedmont, Val d’Aosta, Lombardy). Turin and Milan are the most famous cities, but there are others. Bergamo, Pavia, are all famous gems. This area is famous for the Alpine landscapes of Val d’Aosta and Northern Lombardy, the lakes (Lake Como, Lake Maggiore, lake Orta, lake Como, Lake Iseo, Lake Garda) as well as the wine producing region in Piedmont of Langhe and Monferrato. Some people include Liguria (the Italian Riviera) in this macro region. Primarily for historical reasons, since Liguria (Genoa is the Regional Capital) was part of the kingdom of Savoy based in Turin in Piedmont. You have touched only the eastern edge of Liguria (the Cinque Terre.
This macro region will also keep you busy for a month and more.
EMILIA ROMAGNA and MARCHE. This macro region comprises 2 administrative regions: Emilia Romagna (which is actually two regions merged into one, Emilia and Romagna) and the Marche.
I placed this macroregion together for historic reasons. Romagna and the Marche were both part of the Papal State. Since both are on the same side of the Apennine Mountains they are also logistically easier to travel to. Both have a lot to offer to the visitor and can easily keep you busy for a month.
Once you decide, we can give you more details and reference resources.