I’ve recently jumped into a game of Dragonlance despite the fact that all I know of D&D stems from Solasta & the old Neverwinter Nights games. I came to play an Elf Fighter that I want to subclass to Echo Knight. The GM was so friendly to let me rethink my choice of cantrip, which I humbly seek to be advised on. My current favourite is Booming Blade, which I find mixes well with the ability to move out of the current melee by switching with the echo. It also scales in damage the same way I’d receive extra attacks, so I won’t be missing out on much damage in my view. What would you choose, and why?
Consider a non-combat cantrip. You are a fighter and generally have a more optimal solution in combat using your class features.
Maybe something that fits the theme of your class: Mage hand - fits the echo knight theme of making a spectral hand to grab light things or pull levers. Minor illusion - more illusions and distractions, or mock up a battlefield diagram to share your plan. Message - battlefield commander ability to silently share plans before an attack Mending - maintain your own gear Prestidigitation - cleanup after battle, cool your undershirt on a hot day in armor, warm your coffee at night, start the campfire, show people a palm sized replica of your target/enemy.
Pick something that encourages roleplay out of combat. Give yourself some reason to engage in the downtime action between fights. It will hopefully give you a richer experience.