As far as I know, so far we had 4 limited time events: Mongol Raiders, Barbarossa Brawl, Mangudai Madness and by far the best limited time event so far The Siege (co-op).
Now, thing is, I don’t want The Siege to end. And it would be cool of somewhere after say 6 months every limited time event becomes available in the base game.
There could be a scenario campaign “special events”, but while still making 2vs2 the Siege work 6 months later.