jmr: look at Skyrim. most people wanted to create their own adventures, models, weapons, items, and general character behavior and game mechanics. the amount of mods that were made to be a different game can be counted with the fingers of one hand.
also what do you mean with ‘better off choosing a AAA game to mod for’ ? isn’t UE4 being used for AAA games?
going for the latest UDK to start a new project is still a good idea now, but I doubt it will be so in 5 years.
Chris: most mappers do not need a full SDK, that’s part of the point. in fact they would probably be overwhelmed by it. it’s kinda like having to buy a car when all you need is a bicycle to go to the grocery store 3 blocks away.
again take the Warcraft3 and SC2 editors. you don’t need to be a developer to create maps for those games.