My bet is that the remaining two skills are Security and Mercantile. The reasoning is pretty straightforward.
There are 18 skills in Skyrim. There were 21 skills in Oblivion. We already know the the following Oblivion skills will not reappear in Skyrim: Acrobatics, Athletics, and Hand-to-Hand. (Mysticism has been replaced with Enchanting, and there are some other more "cosmetic" changes, like relabelling Marksman with "Archery". But these changes do not affect the
amount of skills). 21 - 3 = 18. So we've got a list of 18 skills.
Now, if Mercantile is merged into Speechcraft, or if Security is merged into Sneak, then that gives us 17 or 16 skills, depending if only one or both of those mergers occur.
But what are the other skills that are added to those 17 or 16 skills to take the number back up to 18? Here are two possibilities: (i) it's a skill that has appeared in previous games; (ii) it's an entirely new skill.
(i) Let's go through the candidates.
Backstabbing: this is now covered by Sneak.
Climbing: if I recall correctly, it's been confirmed that there's no climbing.
Critical Strike: now a perk in the weapons skills.
Dodging/Unarmoured: maybe, now that Acrobatics is out, there's a skill to govern dodging moves in combat.
Etiquette: governed by Speechcraft.
Medical: resting always fully restores health; other stuff is dealt with by Restoration, Alchemy, altars.
Pickpocket: governed by Sneak.
Stealth: governed by Sneak.
Streetwise: governed by Speechcraft.
(None of the language skills are plausible, I think).
Dodging/Unarmoured is the only plausible candidate, it seems to me.
(ii) I haven't seen any suggestions for new skills which sounded plausible. Probably the best I've seen has been a Dual-Wielding skill; but that's a Combat skill. And we've already got all 6 of those. What we need are Thief skills, if Mercantile is merged with Speechcraft, or if Security is merged with Sneak.
They have already pointed out that probably mercantile and speech will be merged.
No such thing has been pointed out. This is speculation. It has been speculated that Mercantile and Speechcraft will be merged ever since Daggerfall -> Morrowind.