Idea I've had for a while that I've slowly refined in my head over the years.

A new weapon skill-line called Spellsword. It would be one-handed weapons in one hand, and "runes" in the offhand instead of a shield. These runes would have one for each element like the staves: Lighting, Fire, Ice. These runes would simply put a lightning, fire, or ice aura around your offhand instead of having any sort of new item (This completely removes the need to retroactively go back and add a bunch of motifs for a new weapon type.)

The heavy attack would be a channeled ranged attack with your off-hand similar to the lighting staff heavy attack, while light attacks would be melee swings with your one-handed weapon. The resource gain could be cut in half between Magicka and Stamina (so half the standard resource gain could be given as magicka, the other half as stamina). Alternatively, if one resource gain must be chosen, I lean towards Magicka.

Skills would be a mix of ranged attacks (simple lightning bolt, fireball, ice spike, etc.) and melee attacks infused with your rune's element (so similar to crystal weapon just with the element you chose for your rune.) You could also add a "Conjure Atronach" as one of the abilities that summons an Atronach type based on your rune type to give the skill line a bit of flavor and something for us Conjuration fans.

Now would this skill-line be a weapon or spell damage skill-line? That is a good question, and to that I ask another question: Would getting BOTH Weapon and Spell damage break anything? As far as I know most things scale off the greater of the two, but there also could potentially be some sets in the game that would end up being OP if both of these stats were high that I am just not thinking of. If only one stat must be chosen, I lean towards Spell damage, since the skill-line is pretty elemental in nature.

Obviously there are many gaps here for the Devs to fill, and not to mention even without the motifs its still going to be a massive undertaking to add a new weapon skill-line into the game. That being said, I've always felt that the traditional Spellsword playstyle is something that was lacking in the game, and the dual nature of this weapon skill-line would also lend itself pretty well to the recent changes that support a lot of hybrid Stamina/Magicka builds.