Italy has fabulous destinations everywhere, so I'm not sure it is ever possible to say that one place is "the main destination". In addition to the Dolomites, in northeastern Italy you have Venice, Padua and Vicenza, and those are all right on the main train line. If I had only part of a day and was starting from Verona, I'd tend toward Vicenza due to its modest size and proximity. If I had a full day, I'd opt for Padua (which has more to see and is worth a couple of nights). Or you could go to Venice, but you'd be there at the same time as all the other day-trippers, and that's really not the way to see Venice.
What time of year is your trip? A major advantage of the Dolomites is that when you get up to altitude, it's usually very pleasant during the summer. In other places, even in northern Italy (and including the cities along the rail line like Bolzano), summer can be very hot. The mountains are a wonderful escape--which is sometimes needed very badly, because a lot of moderately priced hotels in northern Italy don't think they need air conditioning. This American says they are wrong.