I agree that it is unlikely the Princess Royal will be offered a dukedom - I think it far more likely she will receive some other mark of distinction such as being appointed to the Privy Council (though this is not the honour it was) or receiving the Royal Victorian Chain (she is Grand Master of the RVO).
Also I think it more likely that any future Royal Dukedoms will be granted for life so they don't go outside the immediate Royal Family and this may be the case if, as I think is likely, the Dukedom of Edinburgh is conferred on the Earl of Wessex. This will be similar to the Swedish system perhaps
I have a feeling that, apart from the Earl of Wessex and possibly the Princess Royal, King Charles (and a future King William) may decide not to confer dukedoms or other peerages on members of the Royal Family in the future and the children of the Prince of Wales will remain princes and princesses (which is what I believe Queen Victoria preferred) though with, again possibly, the grandchildren of the Sovereign being granted courtesy titles of children of a duke (similar to the children of Prince Michael of Kent) with consequent changes in the relevant Letters Patent. In which case I would expect that to apply to the children of both sons and daughters of the Sovereign. I suppose though that if the Princess Royal's children can get on in their non-royal lives without titles then why shouldn't other grandchildren of the Sovereign given that, as it seems likely, they will not (except children of the PoW) be carrying out royal duties in a "slimmed down Monarchy" so beloved of the press and the BBC though maybe not quite so beloved when there is a shortage of "Royals" to carry out such duties when, no doubt, the same press and BBC will be the first to complain.
Incidentally there are two Grand Masterships of Orders of Chivalry now vacant - the Order of the British Empire following the death of Prince Philip last year and the Order of the Bath following the accession of King Charles. Maybe the PoW will take the Bath following in the footsteps of his father and Queen Camilla will take the British Empire following in the footsteps of Queen Mary who was Grand Master 1936-1953. Then I would imagine the Earl of Wessex would take the Order of St Michael and St George when that falls vacant.