You Ain’t Gonna Need It
eXtreme Programming (XP) teams use the YAGNI prioritization technique to limit the size of the product backlog. The value of iterative planning is its ability to work on the essential item for the business.
“Welcome changing requirements, even late in development. Agile processes harness change for the customer’s competitive advantage.” – Agile Manifesto Principle
So unless you are sure about the item, do not add it to the product backlog. You can always add it when it becomes essential.
Just as a large queue at the grocery store checkout is disappointing. Large backlogs tend to have a demoralizing effect on the team and the stakeholders. To stay fresh, remember YAGNI.
Here is an excellent article by Martin Fowler on YAGNI.