Well, the main problem with implementing a leaderboard is that A) you need to implement accounts, logging on and off, and get a database going; B)you need to watch the packets when the game sends the leaderboard data, and figure out how it's formatted; and C) You need to figure out how to send the data back to the game, so it can display the leaderboard correctly.
There's probably quite a bit more programming on the back end that you need to do, stuff like managing the database, optimization of search routines, you have a couple choices about how you use a database (SQL, some other database server, or custom data files, which you then need to figure out how to format the file, and then you have to code the stuff necessary to access it, search for the things, add more scores, etc.)
It's no simple task, now that I think about it, but most of the programming is not all that complicated, aside from maybe making an optimized search routine, or analyzing the packets to figure out what all they mean.
I'd be more willing to try to work on this if I could get someone to help me out. If you were to write some of the code in C++, I could translate it into C#, and use it. I could never manage something like this all by myself, but with some help, I could possibly manage it. I'd also need help from you guys to analyze the packets to figure out how it sends things like what character you used, and such.