I'm not trying to rush you or the process. I'd just want some idea what my money was going towards.
As for ideas, I'm not sure the bill board has to say in big bold letters "LEBRON WE LOVE YOU - PLEASE DON"T LEAVE US - SIGNED YOUR FANS".
Something artistic and cool will get attention, and once word gets around how the billboard was sponsored and why - the message will be clear.
A sign begging him to stay would seem kind of pathetic. A sign celebrating what he and the team means to us .. to the city would not. By the way, I emphasized team, because team is important to LeBron. The key would be connecting the things LeBron feels are important with the things we the fans feel are important.
That's the direction I would go, alas I have no artistic skills.
I suppose it might even matter what he and the team have accomplished at the time we want to run the billboard. For instance, encouraging him to finish the job and win the O'Brien may appeal to him. However, if he's already won one an angle might be to encourage him to want to win more.
Another approach would be to emphasize his roots and family in the Cleveland/Akron area. In another city, he'd just be another sports superstar. Here he can put "King of OH" on his license plate and not even garner a snicker because he means that much to the area.