XCom already has its own scoring system, all that needs to be done is keeping a running tally from each mission. I know you get points from more than just missions, so the monthly score could be saved and at the end of the game your total could be displayed somewhere. Maybe the UFOPedia.
A "balance" issue with this would be the fact that, the longer you play, the more points you accumulate. Ideally, we would know the average score that all players get per month, and deduct that from their monthly total. If you do average, you get +0; above average and you score some points.
I think a leaderboard would be another good addition to this game. After I finish with soldier medals/stats, perhaps I could look into "commander" stats, which the leaderboard would be a subsection of.
You can save me some work however by laying out the "rules" of the leaderboard.