Perfect TES, a few points:
-Worlds from Morrowind, Cyrodil and Skyrim
-Graphics and Details from Skyrim
-Dungeons like Morrowind and Skyrim
-UI from Morrowind, World Map from Skyrim
-Music from all above 3 TES
-Skillsystem from Morrowind (5 major and 5 minor skills, long blades, short blades and so on) + the higher the skill, the more you learn and can do (new fighting stances etc)
-Combat System from Dark Messiah
-Elements from Thief 3 (going on for Treasure Hunt at night in the towns

)
-creating and designing your completly own weapons, armor and spells
EDIT:
-instead of Fast Travel as it was in Oblivion and is in Skyrim, use the old travel system like it was in Morrowind, horse stations, trading caravans , silt striders etc