The game would start with the main character receiving a lesson on some basic spells from his master (an opportunity for an in-game tutorial), with whom he lives in a remote cottage in the mountains. After accomplishing the basic tasks in the tutorial, the old wizard receives a visit from a mysterious figure, and promptly packs up and leaves, telling you to take care of things while he's gone. But shortly after he's left, the cottage is attacked by monsters, and after you've managed to escape you set out to find your master and find out what's going on. The plot will turn out to revolve around a rogue wizard who has resurrected knowledge of a forbidden element and is trying to take control of the kingdom. The mysterious figure is an emissary from the Council of Mages, of which your master is a member. The evil wizard is the final boss, and you have to fight him without knowing going into the fight what the symbol is for his forbidden element (you need to know the symbol of an element to cast protection against it), forcing you to pay attention to the symbols he traces so you can learn to cast the protection spell on the fly.
The game would also feature two different kinds of steeds for the hero: a warcorn (a type of tough unicorn) and a falcor (a large bird you can ride).
Unlike most Zelda-like adventures, you mostly don't get money for beating monsters. Instead, you have to work for your pay in different places, but with creative use of your spellcasting abilities you can make many tasks easier. However, you learn early on that you can't let people know about your magical abilities (it'd make you a target of the villain), so you have to hide it.
This whole thing is inspired by some things I find to be lacking in how magic is used in most adventure and RPG video games. Usually, spells are set in stone. In adventures specifically they are used as "another type of weapon/item", no different from a bow & arrow or grappling hook (think Zelda 64's 3 spells) except that they use "magical energy". In RPGs, magic users are basically heavy artillery (sometimes with the ability to heal). I want to turn things on their head so magic, not swords & shields, is primary, and is a flexible tool. The main character is a mage, and the player's tactics have to take that into account.