They can probably cover all the classes with the origin characters by allowing players to select from a short list when the character joins. Wyll, Shadowheart, and Gale seem pretty locked in class-wise, but Lae'zel and could be class changed to most other martials (especially barbarian) without much damage, and Astarion could make a bard or a ranger without much effort.

This is more or less how they did it in DoS 2 (although in Dos 2 the class had less RP flavor for the character).