The only expansion I would buy (and later download, making a downloaded content) is an expansion providing an additional big story arc (the size of one BG3 Act), new side quests, new companion(s), and naturally it would come with shiny new toys like higher level cap, new classes from sources other than the PHB, perhaps even new races, etc.
I would totally consider the purchasing of one class (or class archetype) as a micro-transaction, and I doubt it's Larian's plan to give into these.
