I guess I would keep Skyrim's combat. Spells need some variety and balancing. Skyrim suffers from lack of sustained spells that take percentage of your mana pool for always active magical effects. Casting flesh spells, candlelight or summons is annoying, why not just give players ability to keep them on for a small mana price?
The sneaking system is dumb because there is no possibility to sneak around. You just have series of corridors with no alternate paths. Think of VtM: Bloodlines with sewers, ventilation shafts, back alleys. In The Elder Scrolls it would be translated into rooftops and again sewer system in cities and in dungeons in caves, scaffoldings etc. It would also be good opportunity to add more verticality and platforming from Tomb Raider.
RPG elements and story obviously needs improvements, C&C, faction systems, survival mode, disguises and hybrid world-scaling where weaker enemies are scaled to your level but players have to min-max stats to still beat those with ease, like making build focused around critical hits, crowd control, attack speed, lifesteal or pacification/deception, things that would make light-weight sword master play differently from orc warchief with battle axe.
For a gameplay gimmick, obviously we won't be able to use shouts as that was a Skyrim exclusive thing. So I would just expand on werewolf and vampire gameplay to make it unique. Add some strong bonuses and drawbacks or unique spells/skills.
Or maybe make some spells more gameplay focused. I liked magic from Fable and Kindoms of Amalur because they allowed to make warlocks and spellblades, magic could be used by melee characters and rogues. Think of Become Ethereal shout that works for half second and has 5 second cooldown which allows you to dodge attacks with proper timing while allowing you to dual wield weapons, or Whirwind Sprint which can teleport you around and behind your enemies.
But still without good story, RPG elements the game would be a walking sim or at best a slasher. Gothic is a good example how it should be handled, it's still a good game even with out-dated graphics and atrocious controls.
