This is an interesting question, but without an official source to confirm the answers, they can only remain as speculations.
If you pay attention to the forearm of the hidden character, it is not covered in sleeves, nor is it decorated with anything, like a wrist band, so Kiritsugu is out. Saber's friendly gesture towards the unknown person shows a moderately close affinity between them, so again it can't be Kiritsugu because Saber dislikes him and would have shown an anxious face like Bazett's if it's his hand that is reaching out to her. While Irisviel von Einzbern fits all these bills and is indeed a very likely candidate, the size of the hand seems to be slightly larger than that of Saber and about equal to that of Shirou, which suggests a hand of a delicate man over a hand of a delicate woman.
I think Saber is extending her hand to her male archetype counterpart: Prototype Saber from Fate/Prototype, which was released as a twelve minute anime segment released with Season 3 of Carnival Phantasm.
It makes sense to reveal only the hand because we don't even know of the existence of Prototype Saber until after Ayaka Sajou has called Ahnenerbe in the final segment of the last episode, so the hand serves as a good lead-in to Fate/Prototype.