"Fixed deadline, negotiable scope" has to be the most underrated pattern in product management. It's the secret to shipping.
It's not too hard to come up with a feature. The hard part of product design is sewing many features together into a coherent whole.
The best product development and coding processes preserve your ability to make decisions later when you have better information.
Features you add arbitrarily to a product today become hard requirements in future iterations if even a minority starts depending on them.