See the thread about English bulldogs for info about heat stroke, facial folds, etc.
Also, French bulldogs have strange tracheas, it's very difficult to intubate them for surgeries, so even routine procedures like spay/neutering can be a bit of an ordeal. Some airlines refuse to fly them because they're a liability due to breathing problems. They are also notorious snorers.
Those are the realistic things to consider. On the other hand, all the ones I've been around have had good temperaments, honestly they're not Einsteins but they can be sweet. Just make sure to research all the aspects of the breed, many of the 'extreme' looking breeds are cute and novel looking but they almost invariably come with a set of associated health problems. Then again, you'd be hard pressed to find a breed WITHOUT associated health problems, it all comes down to what you're willing to put the effort into managing.
Good luck with your pup search, it's great that you're researching so far ahead of time!