To digress a bit if you really are into something you should make it your hobby. I am into writing for example. I have a thesaurus, dictionary and one of those clicky ass IBM keyboards. I am living the dream baby. I think of something and just type they shit up right quick. If you are all into something try and make time for it and do it. If you have a family try to include them in it but do it. Go bowling, chop wood, go to highway 5 rest stops looking for anon hook ups with truckers.
The point for role playing being, don't do a theme campaign without letting people know when they sign up. If I was going to run a thieving city based game I would let people know all about it. They could set up their character with that in mind. That ties them into the campaign and gives them something interesting they can do in the vast majority of play.
Say we are sailing around, you could create a sea witch or even just load some wind spells on your wizard. That will make you much more useful and safer than the fireball throwing battle mage. Maybe a pirate build rather than a rogue. All this makes for a party that is part of the game rather than one along for the ride.
There are some themes people are just not interested. I am not interested on being on a boat anymore. but if you let me do something like a semi aquatic character hey I may just be interested in playing. Take Anthropomorphic animal, please! Seriously I just can't go there. I you take my meaning let people know generally so they can decide if it is not their cup of tea. If they are out they may be back in the next game. If they are in and unhappy no one really wins.