What's a good game to create mods for? (besides Minecraft)?

I'm thinking of starting a side career of making independent video games, but I'm considering starting out small by making simple mods for a game and eventually working up to making a total conversion mod. One of the ideas I have in my head right now is to use the Sims 3 or Sims 4 to make some sort of exploration game with direct controls customizable worlds. I have the Sims 3 and several expansion packs, but I did not get the Sims 4 because it seems like a cash cow to me. But again, I'm considering getting the Sims 4 because it is newer and mods would be more popular on a Sims game if it was on the newest one, so there's that. If you have better suggestions, feel free to suggest them. What would be a game with descent (but not too powerful) graphics, yet able to be easily modded?

The two games that have best supported mods have been Skyrim and Grand Theft Auto 4. They both have big modding communities and people who are prepared to help out people new to modding.

Gmod? There are lots of things to do like make player models, items, maps, ect…