Hey there! It really depends on how much u really like the game. I have all expansions and only played with the dragon expansion twice! It's not that it's not good but it's simply because, I also have played the free campaigns in the files section and it has given me so many hours of fun!
Anyways, in short, get all the heroes expansions for the variety of heroes add a lot of diversity in your tactics, play the free summer league campaigns expansions, then decide whether u wanna get the dragon expansions which is basically 4 dragon bosses and variant minions rule.