Yeah, multi-classing is definitely a thing that requires a solid base of system understanding in order to do correctly. I think the game should do something to help newbies along. I'm thinking there should be some function that lets you know when a choice you're making would be redundant with another choice you've already made. Ideally that system would apply the entire time, not just with multi-class, to help ease levelling in general.
As far as players handbook stuff, Larian has explicitly said that they're aiming to get everything from the players handbook into the game, but I highly doubt they're restricting themselves to that. The sense I get is that players handbook content is where they want their floor to be, and they're prioritizing getting all that in but if they can get other stuff in as well, they will. I doubt we'll see other classes or races outside of PHB, but at least a handful of other subclasses seems like it would be doable.
