I do really like the combo damage how gw2 does it...

Things like this should just be a thing, either available as a base status effect, or in the champion point perk tree.

Ice should cause shatter like you suggest.

Lightning should cause arcing damage, meaning lightning will arc between nearby enemies, causing additional cleave damage.

Fire already has a good status proc - burning, which causes additional DOT damage, but in order to ramp up cleave damage (like these other 2 suggestions do), fire damage can come with an "intensify" perk that makes the burning more intense (i.e., do more DOT damage) the more targets that are stacked together.

With the new Class rework, i'm hoping the Dev's will improve the frost builds. In the past frost builds were tank, then they made changes and gave us a good frostbite set, but still, most of what i use as a frost skill is AOE and that is heavily underpowered compared to direct single attacks. I hope Devs will add this "shatter" effect to wardens in the frost line and improve the damage frost does in general.

Lightning is well buffed with direct and channeled sets
Fire is Damage over time buffs and AOE
Frost though needs some serious loving. I'd like to see changes in the frost staff passives. Compounding frost sources to improve damage would be awesome. Something like: for every frost source increase the damage of all frost damage by 1%. I know I can do 10 frost-based sources of damage so that would be 10% more damage applied to each frost source. It isn't game breaking but gives frost a nice boost whether you're using the warden frost class line or not.