I have what I think is a neat idea for a game add-on that I could probably tailor to most genres. The idea isn't a stand-alone game concept, more of an enhancement. As a result, I'm trying to find an existing game that I could try modifying to include it.
What'd I'd really like to see is a system like World of Warcraft's for plugins -- a setup that gives me hooks for a scripting language to make API calls. I can't use Warcraft because the add-on I'd be writing wouldn't be permitted by their servers. Unfortunately, from the digging around I've done it seems that the games that include an SDK expect you to create a new campaign or a full-on mod rather than build onto the existing content.
Can anyone point me to a game that makes extensions easy? Plan B is to choose an open source game and change it, but I'd like to try my idea out on something that has a sizable audience first if I can. Not to imply that open source games don't have fans! :D
Pretty much any imperative/OO development language is fine. If you can think of details I should provide (short of getting into the idea itself), please just let me know in the comments and I'll provide an edit.