Most people now are only playing one class/character because it is to expensive to get gear for alternate characters.

It used to not be as bad with top tier items like Rendtail dragonstone ring and planar pendant adding STR,INT and DEX stats along with primary stat buffs aswell among other stats like crit armor.

Some suggestions are:
Add more items (belts,artifacts,rings,pendants) like those that add stats for all classes and are stashable for alts.
Make craftable energy in events stashable so you can farm on one class and stash for your other characters to use.
Joined stable with all pets shared with all of your characters.
Add jewels that add STR,INT and DEX.

I think these would give people more of a reason to try out a different class instead of sticking to just their main class.