From everything we have seen and read, we can safely accept that there will be eighteen skills. These, we are told, will be equally split into the categories of Combat, Stealth, and Magic, giving a total of six skills in each. The use of perks will further enhance the available skills by providing a unique experience based on your choices as you increase in levels.
As for the skills themselves, under combat, we have been told of two new weapon skills called One-Handed and Two-Handed (which logic suggests would replace Blade and Blunt) and another new skill called Smithing (replacing Armorer, but with the ability to forge weapons) We can safely assume from all of the screenshots that Heavy Armor and Block will also still be available as combat skills. This would leave one available space for either Hand to Hand or Athletics, or maybe something a little different.
As far as stealth is concerned, we know that Marksman is definitely in. We can assume that Light Armor will still be available, as well as Sneak and Lockpick (although these latter two could possibly be combined to create a new "Thievery" skill) One obvious choice for elimination would have to be Mercantile, which will probably be incorporated into Speechcraft (which evidence suggests is also still in) From the original stealth lineup, this would leave only Acrobatics.
As for magic, we have read that Mysticism is definitely out, leaving the five remaining magic schools of Alteration, Conjuration, Destruction, Illusion and Restoration, all of which have been confirmed. We know that Alchemy and Enchanting are both in, but if the rule of six does indeed apply, then only one of these can be classed as a magic skill. The other, by default, would have to fall under either combat or stealth.
According to the Skill Menu screenshot, Enchanting could possibly be introduced as a combat skill, allowing you to add magical effects to weapons, armor and clothing to enhance combat effectiveness, which would tie in nicely with Smithing. This could also perhaps be used to recharge magical items, both player created or pre-existing. The screenshot also suggests, as an alternative, that Alchemy could now be a stealth skill, allowing potions, poisons and possibly even traps to be created to assist in stealthy gameplay as a viable alternative to casting magic.
Either of these changes would mean that two of the remaining skills (Athletics and Acrobatics) would most likely be merged into one skill which would cover all aspects of running, jumping, and climbing. For now, lets call this new skill "Free Running." This could quite easily be either a combat or a stealth skill.
The last skill to be covered is Hand to Hand, which could possibly be removed altogether (leaving unarmed combat as a basic low-level ability rather than an alternative combat system), integrated into the other combat skills via the perk system (which would still cater for those who prefer not to use weapons), or kept in the lineup as a fully fledged combat skill (and probably renamed "Unarmed Combat") If it is kept in, then the previously mentioned Thievery skill would have to come into play to keep the total number of skills to eighteen.
To summarise all of this, the new lineup would look like one of these:
Option 1:
COMBAT: One-Handed, Two-Handed, Block, Heavy Armor, Enchanting, Smithing
STEALTH: Marksman, Sneak, Lockpick, Light Armor, Free Running, Speechcraft
MAGIC: Illusion, Conjuration, Destruction, Restoration, Alteration, Alchemy
Option 2:
COMBAT: One-Handed, Two-Handed, Block, Heavy Armor, Free Running, Smithing
STEALTH: Marksman, Sneak, Lockpick, Alchemy, Light Armor, Speechcraft
MAGIC: Illusion, Conjuration, Destruction, Restoration, Alteration, Enchanting
Option 3:
COMBAT: One-Handed, Two-Handed, Unarmed Combat, Block, Heavy Armor, Smithing
STEALTH: Marksman, Thievery, Alchemy, Light Armor, Free Running, Speechcraft
MAGIC: Illusion, Conjuration, Destruction, Restoration, Alteration, Enchanting
I may be way off the mark, but personally, I think that this is pretty close. What do you guys think?
As a side note, and to bring me back to the title of this (really looong!) post, what would be your "one skill to rule them all" and why? (can be an existing, speculated or made up skill)