To guard against feature and scope creep, insert a clause in the contract for negotiated and agreed changes to the design and development of the game. In the first place, make sure your milestone definitions are as clear as possible to avoid misunderstandings on both sides of the deal.
Inevitably there will be some discussion about scope or that is intended to improve the game. This can sometimes lead to disagreements, with regards to, levels of polish or what should or shouldn’t be included in the price of making the game. Some items you may just have to swallow in fairness, where the definitions are not clear.
Other items can be clearly shown as additional and these should be paid for. Publishers are not naive and always have an additional budget for change control and the unforeseen. You may also be able to make extra money with regards to features, polish and additional scope, if you suggest good ideas to improve the game. If a publisher likes it, they will pay.