What about if, instead of hurling a big stack of barbarian units at you all at once like most events do, if the event spawned a continuous and accelerating trickle of barbs, up to a certain point? Is this not how it was for the Roman Empire? The Roman Empire wasn't destroyed by a single stack of Goth barbarians overnight. It was the constant harassment and pillaging that crippled Rome.
So what if you had a Goth/Hun barbarian event that spawned, for example, 1 unit on the first turn, 2 units on the 2nd turn, 3 units on the 3rd turn, 4 units on the 4th turn, 5 units on the 5th turn, 4 units on the 6th turn, 3 units on the 7th turn, 2 units on the 8th turn, and 1 last unit on the 9th turn. You get a visit by the first roaming barbarians, who bring word of great migrations afoot. This gives you time to prepare. Then the units start coming fast and furious 25 units total over 10 turns), and you don't even have time to heal before the next wave hits, meaning you have to be pumping units out like crazy. This would be more fun, I think, than the standard "You lose!" card that you get dealt with having 4-6 Vedic Aryans or HA's suddenly appear 2 tiles from your city.
The alternative would be to have all of the units spawn at once, but have the mega-stack of barb units be required to spawn a minimum distance from any city (like, let's say, 8 tiles). Having the barb events spawn "outside of borders" doesn't cut it when in the early game your "borders" amount to only the first or 2nd rings. If you stipulated a reasonable minimum distance, you might be able to allow for a 10+ unit spawn in this case without the event being an automatic game over (because in 4-8 turns you can produce and send to the front quite a few extra units with slavery).