There should be more companions coming to the game, yes.
While datamining should definitely be taken with a grain of salt, and not everything that is in the datamining will end up in the game, and plenty of things that aren't visible in current datamining at all will end up in the game, I doubt very much that a Baldur's Gate game will be limited to just the 5 companions.
I'm sure there will be more companions to be unveiled yet.
