Also sim games, where you can build a world or city or house or country or whatever. What kind of games allow you to create things that you enjoy showing off? What kind of games have things in them that interest you enough to want to see someone else's creations?
And lets not forget puzzle games. The Incredible Machine, Chu Chu Rocket, Lemmings, ... Uh... and others. Those are pretty easy to add content (levels) that could help create a community.
A sim game would be easy to extend, but hideous to get started.
A puzzle game on the other hand would be nice, but they generally exhaust their level ideas fairly quickly.
Oh, and please don't double-post.
Sure, it's probably fairly trivial to add a new kind of tower or enemy type in a TD game, but the hard part is in the balance. Some problems that plague TD games are that (1) you could beat them using just one or two kinds of towers. (2) One strategy worked on every level. (3) Or another problem I've seen a lot is with the difficulty (which also has to do with balance, IMO). You'd be doing great for 86 waves and then suddenly it became impossible.
Why not have the game choose a random subsection of the total number of towers at the start of the level? If you don't have the Flamethrower tower all the time, you can't possibly use the strategy based around it.
And if it does get impossible, why not simply design a few waves in the middle, or make a tower that might be useful in that situation. Of course, balance is always going to be an issue, and creating an overpowered tower would definitely be an issue.
I do have a solution around here somewhere though.
A while back, I started work on a procedurally generated TD game. The computer would score the power of your setup, based on a combination of dps and how well the last wave fared. It would then generate a wave which is slightly stronger than it thinks you can handle. I designed it based on the L4D2 Director - It targets weak points, and subtly throws wave after wave against them (I had a game where it threw five or so flying waves in a row,and then a fast ground wave when it realized I was onto it)