Free main hand with an offhand weapon

[Aaedimus]

So instead of dual wielding, or using a shield while casting your cantrips, I use my main hand to cast spells, and carry a scimitar in my off hand that I can't add my proficiency modifier to for the bonus action attacks.

Anything stopping this from being a thing?

[Spectrulus]

You can also just cast spells with your off-hand while adding your relevant stat to attack rolls. Most DM's are totally fine with casting while using a two-handed weapon. Why would you want to only use your off-hand?

[JNAProductions]

You can only make a TWF Bonus Action Attack when you use your main action to Attack. A cantrip does not qualify.

[Unoriginal]

5e doesn't have rules about which is your main hand and your off hand. It's just "the hand you're currently using to attack the most" and "the other".

[Spectrulus]

If you want to attack while casting spells, Eldritch Knight can attack while casting cantrips at 7, and while casting spells at 17. These attacks can even be with ranged weapons if you were so inclined.

[sophontteks]

You aren't taking the attack action and you aren't attacking with a melee weapon, so you can't bonus action attack.

[Xihirli]

So instead of dual wielding, or using a shield while casting your cantrips, I use my main hand to cast spells, and carry a scimitar in my off hand that I can't add my proficiency modifier to for the bonus action attacks.

Anything stopping this from being a thing?

TWF takes your ability modifier from damage, not your proficiency modifier from to-hit.
Also all the stuff about that not being the Attack action.

[Vogie]

So instead of dual wielding, or using a shield while casting your cantrips, I use my main hand to cast spells, and carry a scimitar in my off hand that I can't add my proficiency modifier to for the bonus action attacks.

Anything stopping this from being a thing?

Yes. The Rules.


Two Weapon Fighting
When you take the Attack action and attack with a light melee weapon that you’re holding in one hand, you can use a bonus action to attack with a different light melee weapon that you’re holding in the other hand. You don’t add your ability modifier to the damage of the bonus attack, unless that modifier is negative.

There are no "Main" hands or "off" hands
If you're wielding a weapon in either hand, you are wielding a weapon.
You'd only get your proficiency modifier if you are proficient with that weapon. For example, non-Stone Sorcerers, non-Hexblade Warlocks & non-Bladesinger Wizards won't have proficiency in Scimitars, period.


Now, if you were homebrewing a class or subclass and wanted to replace an Extra Attack feature with War Magic (EK feature), I think that'd be pretty cool. It works with EKs because the fighter class has so many attacks already, so choosing to use War magic & Improved War Magic is a meaningful choice.