I've never worried about multiclassing and wouldn't be concerned with additional classes causing balance issues since I don't really worry too much about balance between classes/multiclass combinations, they just need to be balanced enough.
I expect there are a lot of class concepts that I'd love to see an official take on that won't get made, but as long as they make any required classes for a setting, much like the artificer was made for eberron, I'm not too worried. That means I'm hoping for a psion for any potential dark sun setting book, though I'm halfway expecting them to create some sort of module that you attach to current classes similar to the supernatural gifts/piety of Theros but with uniquely psionic flavour.