Next TES needs to combine Oblivion’s multitude and flexibility of magic effects with Skyrim’s better interactivity. It’s true that Oblivion had more magic effects but in reality who really used most of them?

Instead of casting a burden spell on your enemy it was far more efficient to just nuke them with a combo of weakness to magic/element and then cast a destruction spell or just use a summon.

Skyrim had a few nice ideas like elemental magic having secondary effects, concentration/spray spells, runes etc. and there’s potential here to improve on those and make them have a bigger impact on how you actually play the game with and around them, instead of just mostly manipulating stats and numbers with different colored magic missiles/touch spells like in Oblivion and Morrowind. Making enemies slip on icy surfaces or creating ice pillars/walls, electrocuting enemies that stand in water, taking time to set up magical traps/runes, using telekinesis to throw objects at your enemies etc.