Functionality creep

Scope creep (also called requirement creep, function creep, feature creep, or kitchen sink syndrome) in project management refers to changes, continuous or uncontrolled growth in a project’s scope, at any point after the project begins. This can occur when the scope of a project is not properly defined, documented, or controlled. It is generally considered harmful.

Scope creep can be a result of:

Scope creep is a risk in most projects. Most megaprojects fall victim to scope creep (see Megaprojects and Risk). Scope creep often results in cost overrun. A "value for free" strategy is difficult to counteract and remains a difficult challenge for even the most experienced project managers.

...
Wikipedia