Yeah, there are a lot of tweaks one could do to the "weapon" traits. It would be nice if there was a line of traits for ranged weapons as well, instead of just tacking the one trait, "Rain of Arrows," into the Assassin line. There is room for something like a "Double Shot" for bows, or arrow attacks that knock prone or slow, etc. I also feel like the other non-mage classes should get some attack bonus skill choices as well, although perhaps not quite as high as the Warrior bonuses.
But, to a great extent, I think we have too many "classes," especially when you consider the limitations we have in picking from 2 Champions each time we get a choice. I think the game would be best served by reducing the classes down to 4 and combining skills. The Warrior and Defender could really be combined into 1 class, and allow you to figure out how you want to advance the warrior--into a more defensive champion or a more offensive one. There are a number of superfluous traits in the Defender line, and I usually skip Defenders whenever given a choice.
Commanders are useful, and I often take one early if I have Bacco because the bonus research is really helpful. And, late game they are often the best choice.
Mages are, of course, very useful.
One tweak I have been trying out is upping the Adventurer's Guild somewhat. At the moment I'm trying it at 5xp per season and it seems to help boost heroes who are just sitting around late game so that they can get a few traits. Another option would be for the Adventurer's Guild to provide an xp boost to champions stationed in any city.