This is a good suggestion. Dipping is kind of weird currently as you can ignite a metal blade with a candle. If you had to coat your weapon in grease before hand or dipped it in oil and then set it on fire it'd make more sense and be more like a fun creative choice a player might make in DnD.
"I rush to the vat of oil and plunge my sword in! Then I take my torch and set it on FIRE."
"Congrats, you now have a flaming sword for a few rounds."
