More Elder Scrolls, less Fallout 4. I didn't like the modernization "improvements" they made with FO4 at all. The cutscenes and loss of control wasn't great. It did have some features that could be cool in ES, like weapon and armor customization. Stick to the style that has worked so well, open world choose your own adventure.
More options in the skill trees. I'd like to see hybrid skills, like if you reach a certain level of archery and magic and also have a magic skill, you can shoot split arrows like Chebacca's bowcaster.
Enchanted weapons to return to Morrowind style, where they recharge over time if you aren't using the weapon. It would encourage trying new weapons or combat styles, and it would ditch the horrible using soul gems to recharge your weapons system. Enchanting is my least favorite part of Oblivion and Skyrim.
Expand the home building they introduced in Skyrim. We don't know what the story is so it might not make sense unless it's a dlc, but I always love the building up a fortress and team in games. Let us build up a fortress and put specific buildings that offer specific bonuses to our defences, and have NPC armies raid it. A first person tower defense, kinda. Though it was really simple, I loved that part of Neverwinter Knights 2.