My take is once the campaign is completely developed, the level cap will be set in stone. It's really hard and takes a lot of work to come back to a completed thing and add more to it, thus increasing the level cap after release would be a more difficult endeavor than adding subclasses and races after release. So I'd say the level cap would be my priority, since it wouldn't be as much to add the subclasses/races later. I am completely ignoring the possibility of expansions, of course, but I'm generally not a fan of expansions since they are usually quite underwhelming compared to the main game, depending on the studio. Plus Larian seems like they prefer to improve upon their base game after release, rather than go the expansion route.
Granted adding subclasses/races would have implications throughout the story, such as unique dialogue options or racial abilities that could circumvent encounters in different ways. But I think this would overall be a smaller effort.
