I would love something like this! I think like, exclusive items that you get from games would be cool especially depending on the month you get the carnival to show up, like if you got it in the spring you get flowery or plant-ish themed furniture (not exactly the weeding day furniture but something for the spring time) or even flowery clothing like a flower crown.
I think having more events like a carnival would be really nice... I think something along the lines of how redd and katrina's tents are, or the camping villagers but more fleshed out of an event, but ya know not as intense as the actual holiday events.
I think having like a separate area like the camp grounds in the update for these types of things would be really neat. They can have exclusive furniture or items that you can only get there, or allow certain villagers even to move in, maybe specific flowers or bush starts.
Or more areas like the city or island that you can visit for the same effect. I think a mix of random events and special places to go would be really cool, keep it fresh, and give you more things to consistently check on in game. I almost want too many options so that it always feels like i have a goal or something to look forward too. As well as would give your specific experience with your town a completely different play/feeling to it than someone else's