I like the concept of dipping/coating, and I'm perfectly willing to suspend (some of) my disbelief in exchange for the coolness of a flaming or poisoned weapon.
But a strong +1 that the mechanic should be relatively balanced, and be a bit more realistic than it currently is. Dipping/coating shouldn't be the free-material + bonus action cost that it currently is. To remain a bonus action, it should cost a consumable (oil flask, alchemist's fire, poison). Then it'd make sense (and remain somewhat balanced) to ignite an oil-coated sword by dipping it in a fire source.
Perhaps oil flasks could be less expensive, but require an additional dipping action to ignite (can you pre-oil a weapon before combat?).
Whereas Alchemist's Fire flasks are more expensive, but automatically light the weapon with their use and the flame lasts longer.
