Eh, the game already does this. You don't instantly lose your momentum when you crouch. I could make the momentum decrease slower, but a lot of people use crouch as a means to slow down, like when you're trying to land on a small ledge. With sliding, you'd slide right off your target. I'm not sure what this really gives you, anyway, besides maybe a mild "neato" effect. It seems like it would just make the game movement harder to control.