Rule of thumb for player input; ideas and questions are great, but pushing is not. 

For example, I run an RP with quite a lot of fantastical elements, but it's a setting with only humans as and where castes don't matter. I've had people who wanted to create elves or nobility. I didn't want to deal with race balancing (running a stat-system makes attracting players hard enough, so it's kept as simple as possible) or characters being viewed as more important because of their birth (would bring in an unwanted social dynamic between players.) Some people managed to work around that, like having a summonable spirit with elvish features. I even had a player whose family brainwashed itself thinking their family name meant shit in the world (but OOC acknowledged it didn't.) ie, adapting an idea to fit with the pre-enstablished borders was fine. I've also had players who kept pushing for elves and nobles after telling them why they couldn't. Those were outright declined, because they didn't want to fit an idea into a setting, they wanted to force an unwanted change in the entire RP.