Blademage

You have mastered the ability to accentuate your attacks with magic. Your training grants you the following abilities:

• When you take the attack action and use your extra attack feature, you can cast one of your cantrips in place of one of those attacks. 
• You learn the Truestrike Cantrip. Additionally, you can cast it as a bonus action a number of times equal to your proficiency bonus. You regain all uses at the end of a long rest. 

(Design Note: The extra attack feature is designed to be compatible with the Bladesinger 6th level feature. A Bladesinger with this feat could replace both of their attacks with a cantrip.)