Discussed a lot of times before.
It wouldn't work right now, mostly because the game isn't very huge. Conventions that are game-specific are usually for things that a good 75% of people have heard of. That's why there is Brony conventions and Otaku conventions but not osu! conventions.
The population isn't as huge as it seems. Even though it says there are 4.3 million users, there's usually only about 10k users on at a time. There's more than just 10k active, but still, it's not the 4.3mil that are always on.
Then take into mind that the population is pretty far spread. From my home area there's a total of 1 person that I know that actively played osu! (And the Nearby function only gives me like, 5 people in total. Not at the same time, tho.). At my college, there was only 1 person who played the game, and he quit as soon as he met me since he didn't want to be associated with things that I liked. There's more in big cities and other countries where the population isn't so far spread, but it's still not neccesarily huge. I can mention League of Legends to most people in my area and they'd have heard of it.
Then take into mind the amount of people that would actually go to a convention. My parents are stingy and wouldn't let me go anywhere until I turned 18 (and even then they're still a bit stingy). A lot of younger parents are like this, especially when it's something like "OH HEY LET ME DROP OFF MY KID AT THIS PLACE LOL." A lot of the active population are younger members. So you'd only get some older people to drive. But older people need gas money or money to stay at a hotel, so there's less people that will go.
Then take in mind, what exactly would there be at a giant-ass convention. Granted, there's not always a lot to do at conventions, but you still have to think about "Why would I drive all the way out here to spend my day here". Even if it's not entirely all organized activities, there still needs to be something.
I'm putting way too much effort in this and thinking about what there would be to do.
1. Probably a visit from peppy to talk and be like "All of you guys are assholes"
2. Maybe something about osu! arcade.
3. Pro players that would come down and do shit (Which would have to be done entirely out of charity, more than likely, since osu! doesn't really make that much money)
4. idunno some tablet shit
5. furry rave
6. I'm legitimately out of ideas
Then after doing all of that, factor in the cost of renting out a facility to have an event, and have the cost of tickets be high enough to make money but not so high that it leads people away (Keep in mind that making it cost money at all will lead a large group of people away.)
It's not worth it.
Cons usually do successful either because there's a huge fanbase that the loss from the cost to enter won't affect them too greatly (video game cons, anime cons, etc) or because it's the only way a large group of people can interact, so they're more motivated to save up to go (anthrocon, bronycon, etc).
go do stuff with people around the area if you wanna do stuff, since osu!cons are highly unlikely