The problem is with balance, not with visuals, because those can be addressed easier with animation changes or styles.
When everyone picks the same multiclass combinations, like everyone has an arcanist beam, there is no more class diversity, there is no problem to solve with your class, you just follow the train.
In a class system, each class should have unique benefits that feel rewarding, like I can't vanish, but I have this powerful shield, or I can teleport, but I am not good at heals etc. Getting such a build right, and making it rewarding, is what made this game interesting until some years ago, when there was a lot of talks about "homogenization", "streamlining" etc to allow hybrid builds. People like options and sandboxy games, so now it's very hard to do a U turn and say, we redefine classes like this and that. It shows the devs don't really know what they want, and they had many takes on how to address these issues.
In my opinion, they'll never get it right, the cat's out of the bag, unless they do something dramatic like remove multiclassing, which is unlikely. The least I would like now, is a range of visual effects, so at least I could create a visual theme around my multiclassed and broken chars (btw, I switched back to simple class on my main).
In ESO classes don't bring unique useful things in a group that are memorable. Some people compared to WoW, where mages created portals to cities or food/water or a table with food, warlocks could summon a group of people, paladins had the invulnerability bubble etc. Maybe having such unique abilities tied to your class, and losing them if you multiclass would make some players stay in the class, but the choice must be rewarding beyond some situational perk.