Mainly Agility and Strength for attributes, acrobatics and mysticism for skills..

Was that possible in oblivion?

I get the feeling that attributes still exist under the chassis of this game, and that making them visible to the player might be fairly easy.

As for adding in new skills, that depends on a lot of things. We will almost certainly need a script extender for that because in order to really integrate a modder scripted skill into the game it needs to show up in the menus. I thought it'd be impossible in Oblivion but then MenuQue came out and changed how I saw the issue.

You forgot Wisdom and Intelligence for mages etc

You can bring them back.. The complicated part will be blend them with the system. And them blend it which others mods. It's possible, just plain hard.