1.) Make this a wizard subclass.
2.) At 2nd level they gain martial weapons, medium armor and shields. They also gain a single elemental power they can emanate from their weapon (flaming sword, etc...) for an extra d4 (that grows to d6 at 6th, d8 at 11th, and finally d10 at 17th). In addition to lighting on fire, they'd also be able to do something (cold slows 10 feet, fire provides light and can ignite, acid does double damage to objects, etc... Additionally, they gain temp hps whenever they cast a spell (to make up for their low wizard hps).
3.) Give it access to unique 'spellstrike' spells at lower levels similar to the smite spells of the paladin. Also allow them to concentrate on two spells at once when both spells target only them.
4.) At 6th level, give them the ability to cast a spell as a bonus action, but only on a target that has been hit by their weapon attack that round. They would not get multiattack. They also gain heavy armor here.
5.) At 10th level they get resistance to damage from spells.
6.) At 14th they can concentrate on as many spells as they wish, so long as the spells target only them.