Up until the last decade-and-a-half, the twin concepts of theme and game mechanism were pretty exclusive of one another. If you wanted to play Call of Cthulhu with the D&D rules, most people wouldn't really care (but the D&D grognards might tell you to play "Ravenloft")...in the same way, TSR was producing numerous variant skins for D&D, each using the same basic mechanical structure but simply overlaying a new gloss. The same thing happens outside roleplaying with variant boardgame versions...Monopoly, Star Wars Monopoly, Dr Who Monopoly, Toy Story Monopoly...the same game, just with different gloss in the hope it will sell a few more units.
In the world of RPGs, places like the Forge highlighted the disconnect between theme and mechanism. Suddenly it looked like we were doing it wrong. New waves of game design have come through, but I still see the dramatic disconnect on the shelves of department stores when looking at board games. My recent unveiling shows that most people just don't see the issue...when I tried to personally explain how games can specifically address themes rather than just having a theme "stuck-on-top", it came as a revelation. People were amazed by the concept of a game that was designed to teach more than just mathematical skills or spelling ability, it was designed to reveal something through convergence of mechanisms, it was pointed toward a specific play experience, and it pulled deeper emotions and abstract concepts into the rituals of play without blatantly stating as such.
I pointed to Monopoly as a game where emergent play creates an experience very different from the flat reading of the rules...in game design blogs I've seen the example used time and again. The rules don't state "friendships may be lost over this game", and they certainly don't state that "the best way for everyone to get enjoyment from monopoly is for everyone to avoid play", or "as soon as anyone buys a property, it's all downhill toward one winner and many losers". Monopoly is cut-throat, it's a good reflection of the corporate world. But Star Wars, Doctor Who and Toy Story don't mesh with that paradigm...they are simply marketing gimmicks slapped on top.
I thought about adding new themes to "Bug Hunt". The basic premise is girls chasing bugs or butterflies across a swamp/field, but it could easily be reworked as police chasing criminals across a city, prospectors trying to find minerals or mine sites, pirates looking for treasure...each of these theme ideas can be added to the game quite easily, and they expand the potential audience of the product, but do they improve it in any way?
I've still got a lot of thought to do on this topic.