Possible? I would think so.
Going to happen? I am not aware that Purism has said anything but it would be hard to believe that it is not something that they would at least be thinking about. I believe that they were previously doing something with a tablet but that maybe that went “on hold” when they started work on the Librem 5. Once the L5 is born, hopefully they will pick up the tablet project. Assuming that the L5 is a success, I would definitely be a potential customer / backer for a tablet (Librem 10? - yeah I’m putting in a plug for a 10" screen).
See also: Your Purism products wish list if you wish to express interest.