You say it's past time for a time travel episode, but I say its PAST TIME for a time travel episode.
This all started because of personal loss. Magic was marvellous, until Uthur made a deal without reading the contract and suddenly his wife is dead. Then he spends 20 years building pyres to get over the fact he fucked up.
He transitioned his guilt into a heady body count upward towards 6 digits.
A time travel episode would give Arthur and Merlin a choice, keep everything as it always has been preserving the timeline, or sacrifice Arthur to side step all that ethnic cleansing by getting inbetween Uthur and Nimueh.
What would have been the negative consequences if Ygraine lived?