This is a topic where you can post your ideas of dream addons that could give roleplaying, building, or simply hanging out with friends a big facelift, but could be impossible for you to do without the necessary scripting, modeling, or animating skills, or would have any legal issues to them.
Here's one that I thought up.
GMod Arcade / Retro Games - This idea is all pretty much revolved around ROM's and Emulation which certainly isn't the most legal thing on the planet, and would take some serious programming to get onto gmod. You could spawn and play arcade games, game consoles, old home computers such as the Commodore 64 (like PC Mod but emulator-based), handhelds, and probably pinball machines (Pinball games would need far more work than video games due to being physics based) In-game.
Example - You could spawn an arcade/pinball game, walk up to it and press E and you can play it in-game, The same applies for consoles and computers with the exception of plugging the system into a monitor then loading up / inserting a game and contollers in. Handhelds will work in a similar way with them being equippable SWEPS.
Here's a basic gmod pic kinda showing what this would look like when one is playing International Karate on the commodore 64.
This could give roleplaying a decent expansion, people can run arcades, sell consoles and games, or simply add an arcade game to their business for some small profit.