Well, depending on the game, i'll gladly help you to code the stuff you need for a mod. Actual game development would be new land to me, but I've wrote several mods/fixes/tools for different games over the years. Most notably a whole slew of mods for WoW (back in the day, where it was actually fun to play). More recently I've provided several fixes and conversions for Skyrim, Fallout 3 and Fallout New Vegas.
My bread and butter is coming from web-development, mostly PHP, and smaller desktop applications. So of there is any thing i can do for you, let me know.