I am happy both for PoE2 being made as well as multiclassing making the cut.
If single class characters were to be treated closer to how multi class characters will work a lot of the balancing issues can be sidestepped.
If you were to add a second duplicate of each class which either is only available to main class characters, or as a virtual second class for single class characters.
The comparative progression of single versus multi class characters would balance out more nicely.
Consider the following example:
Class/Power source + Class/Power source
Wizard + Fighter -> Battlemage
Wizard + Wizard2 -> Wizard
Were Wizard2 could contain more horizontal progression for Wizards, and/or unlock a few perks/talents only available to single class characters. (Though this might not be necessary, and is only an additional suggestion.)